On some parts you'll find numbers / letters / symbols or a combination of these that have been cast into it.
And other parts have no casting numbers / symbols / markings at all.
HOWEVER, IF A PART HAS A NUMBER CAST INTO IT, THAT NUMBER IS NOT A PART NUMBER.
Some of them will look like a part number since they have the same -xx year code on the end as part#s in the catalogs do.

HD uses outside vendors (suppliers or manufacturers) to build many parts found on HD models.
The vendor may have been the one to originally cast or stamp those numbers in the part.
And HD may add their own stampings also before the part is fitted to a motorcycle.
So the casting or stamped numbers / letters / symbols etc. found in a part will denote many things other than part numbers.
Basically a casting number details a certain main design of a part.
That design may require other machining (holes, slots etc) before being classified as a part to be used in production.
The main design may simply require different finishes or colors for different models before being used in production.
The main design may also require other parts added (bearing race, sleeve for holes etc) before being used in production.
When the main design changed, so did the casting numbers, but not until then. However, the part number may change several times.

Some parts will be identical when they are originally made by the Moco or vendor or whoever.
Then, one step toward the part's final production state may be adding a mounting hole for one model.
But another model may not require one or may require the hole drilled in a different location.
Some parts are different in either having a chamfer or not. Some are only different in color or texture.
These will have the same casting numbers but due to other changes made before final production, each may have different part numbers.
Likewise, different part numbers (intended for different makes or models) may actually have the same casting number.

Other than Ebay or other online sellers without access to the actual part number, you will not find the casting number in a parts catalog or retail listing.
Many parts are also sold on Ebay or other using the casting number presented as the part number.
And sometimes the casting number is the only way to be sure the part is right for your bike (since part numbers are NOT cast into it).
So doing a search for the casting number (if you have that info) may give you better results than the part number but not always.
Many sellers will also pull a part off a bike and look up the part number (that came from the factory on that bike).
Unbeknownst to the seller, the factory part may have been swapped out for a different year part before the seller got the bike.
But without proper verification, neither the buyer nor the seller can always be sure the parts are correct as listed.
So beware of accepting the part based off that criteria.
There are plenty of bikes that were never molested in there lifetime so it can be a guessing game whether a part is original or not.

AGAIN, CASTING NUMBERS ARE NOT PART NUMBERS.

• 1952-1980 K, XL cam covers used casting number (25202) with a dash number on the end but part the numbers changed app. 20 times.
  The dash number on the end changed only 8 times.
  57-62 had a (-52A) casting number and were part number (25200-57). 63-67 also had a (-52A), made from different material and were the part number (25200-63).
  So they had the same casting number but separate part numbers. Reference 57-85 Gearcase / Cam Covers
• Heads have casting numbers that will confuse you especially if you have a 2006 motor with (-02) head castings.
  
• 1986-Present rocker boxes have casting numbers that remain the same although the part number changes many times.
  The casting numbers changed very little but the part numbers were numerous. Reference Rocker Box Part Numbers
• Casting numbers come in different patterns and some have no patterns at all.
  • The casting numbers that usually fool us are the ones that look like part numbers (XXXXX-XX).
    The dash number on the end is misleading in most cases. A (-92) dash number is ASSUMED the part was made FOR 1992 models.
    But the truth is the (-92) is the first year that certain pattern was made.
    And that same pattern of part may be used for the 5-10 years with other changes for to the part for different model bikes.
    Reference Evo Rocker Box Cast Numbers and Date Coding
  • The year of manufacture of a part is sometimes cast into it and sometimes with the month included.
    A circle with the last two digits of the year in the center is a typical year code casting.
    On some parts the circle also has sections (or pie slices) cast around the year code.
    Some of the “pie slices” will have a dot cast into it. Count the number of slices that have dots in them to find the month the part was made.
    (8 slices dotted around a “94” cast into the center represents August of 1994 production date)
    Some go further with another circle code casting that may denote the day of manufacture.
    Reference 1991-up Oil Pump Date Code
  • Other numbers / letters cast into the part can be control numbers or other.

Many castings numbers have been cyphered out in the Sportsterpedia in the respective part sections.
Links to that information in the Sportsterpedia are provided on the bottom of the examples below.

• Casting numbers can be used to identify SOME parts or year models used on if you can decode the numbers and this fits your criteria.
  • The casting numbers are sometimes arbitrary between year models and can't be used to date the part.
    So consulting a verified list of casting numbers can help in decoding the parts.
    • Example: No date codes were cast into 52-85 cam covers but were cast into 86-up covers.
      The first 5 digits of the casting number on 1952-1980 cam covers are (25202).
      The -dash number for 52-56 K models is (25202-52). The -dash number for 57-67 XLs is (25202-52A)
      But then the only casting number on 68-70 covers is (02-68). 71-76 covers used (25202-71)
      Then they throw another twist with the -dash number not inline with the first five numbers (76-25202F) and the -dash number is above the five numbers.
      So knowing ahead of time the pattern of these codes can be important in find what you are looking for.
      Reference 57-85 Gearcase / Cam Covers
  • However, building a data base of casting numbers is not the only way to decode them depending on what you are needing to know.
    If the part has a date code, you can sometimes deduce what year the part was made.
    This can, but not always, reveal what year the part fits (not particular model though).
    Sometimes it takes the casting numbers along with known features or other markings to decipher what the part fits.
    • Example 1: 1957-1969 models use casting numbers to identify certain parts of the frame for year / model ID.
      57-69 models did not have a VIN number stamped in the frame. A serial number was stamped into the motor.
      The only way to ID what year and model frame you have is to find and decode the casting numbers and known frame features.
      Reference Ironhead Frame Identification
    • Example 2: Casting numbers can be important when buying a used carb with the correct internals for your year model.
      Keihin CV carbs have casting numbers and also the part number is on a sticker attached to the carb.
      The stickers make it relatively easy to match the carb to what year model it was fitted to from the factory.
      But sometimes the sticker goes missing (common after the carb sits in carb cleaner) or is unreadable.
      Then you'll have to depend on the casting numbers and / or internal inspection.
      However, casting numbers (only) were cast or stamped into early carburetors.
      These markings identify type, body, jetting, motorcycle used for or modifications done to them.
      Reference Links from the main IH Carburetor Page

Many parts have a circle date code cast into them. This refers to the date the part was made (not the year model is was made for).
Generally the date code will be inside a cast-in circle with equal division blocks around the date in the center.
The division blocks will (most of the time) have dots stamped or blotted into the blocks to represent the month with the center number being the year.
Knowing the year season helps a lot. Generally you can look to June or July being the start of the next year season for production parts.
There most likely will be a layover between ending season and new season parts however.
So those two months are just a general point.

Example: If you have a cam cover with a circle date code (90) for the year 1990, it could have been made for either 1990 or 1991 model applications.
A circle date (90) and three division blocks blotted would indicate the part was cast in March, 1990 (late 1990 model season).
The part will generally be made for a 1990 bike.
A circle date (90) and eight division blocks blotted would indicate the part was cast in August, 1990 (early 1991 model season).
The part will generally be made for a 1991 bike.
Then compare features or known casting numbers, if applicable, for a positive year match.