Click Here to reference Sportster VIN Information in the History section of the Sportsterpedia.

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1952-1976 frames will interchange. Short frame was 52-66 all and 67-69 CH. Long frames were the other years / models. ¹⁾
To change from one to the other you need: frame, swing arm (also long or short. It's the swing arm that varies the wheel base), driver peg supports.
Using the short frame with hump (long fame) motors will require some kind of solution of side stand contacting primary drain plug and boss.
The primary drain is a hump motor thing. To integrate it's addition to the cases, the foot peg mounting points and kickstand tab were moved forward on the long frame.
Hence the different driver peg supports to bring the pegs back into position.

This is a quote from Moon Wolf regarding short frames:

The XLH and XLCH frame are essentially the same frame thru 1966. ²⁾
The XLH has some extra mounts but you could make an convincing CH frame out of one of them quite easily.
The “dogleg” H or “long” frame debuts in 67, and provides additional room for the electric start apparatus, which also debuts that year along with the one inch longer swingarm.
Whatever language you choose to use, the short frame and magneto more or less signify what we guys from the 60s think of when we hear the term.
In 1970, the CH gets the long frame and a timer in place of the mag–two of the reasons some old timers feel that 69 is the last year for true CH.

The long frames have a spot on the seat base for the under seat ignition switch (short frames don't have this extra wing on the seat casting). ³⁾

From Moon Wolf on modifications. ⁴⁾
Modifications were very common (on these bikes). I would guess about half the frames you run across have the ears cut off.
In the sixties and seventies, everyone was running chopper seats and the ears got in the way of the flat look we were after, so we hacked them off (sorry).
In those days, we were anti-brackets and anti-protuberances of any kind, so in many cases the large tank-mounting-boss below the ears went with it.
(along with the front boss for the damper, crashbar mounts and so on)
The guys that did so weren't idiots, they were just doing what kids do. Modifications often started the day you brought your new bike home.
As for me, I know that in 1969, when I bought my 1961 XLH, it took me about four weeks to make it into a near perfect XLCH clone.
(fenders, tanks, shocks, headlight, and magneto and then another year to turn it into a full blown chopper)

The distance between the seat mounting ears is 1-5/8“. The inside of the ears line up with the outside of the main tube.
The bottom of the ears line up exactly with the seat casting. Measuring from the end of the main tube to the tip of the ears is 7-5/8”. ⁵⁾

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