There was a truck load of changes in the horn mounts between late 1970 till 1975. ¹⁾ The changes weren't really cataloged in the part books.
Basically:
1965-1970 all used the same geometry. 1965 and 1966 used different stuff than 67> but the end result was same.
And this 1st gen stuff was used on some 1971's, maybe even some 72's also. This set up used the larger diameter, short, stiff, 5/16“ stud rubber mount.
It held the horn way more solid than the later smaller, longer, 1/4” stud rubber mount. Both 1st gen brackets had less inward offset than any of the later ones.
The shananigans began in late 1970 with the intro of the 'low' seat kit.
1971 and maybe the 1972 bikes that came from the factory using the older standard seats supposedly came with the 1st gen 65-70 setup.
In late 1971, the low seat was available factory installed for the 1st time.
The low seat required the ignition switch to be moved from the right side under the high seat to the left side on top motor mount where it fouled the horn brackets.
At first, the new brackets with deeper offsets that opened up room for switch were supplied that used the old 5/16“ rubber.
There were some issues with interference. The smaller rubber mount and another set of new brackets were furnished to solve that issue.
But the 1/4” rubber mount let the horn flop around to much hitting fins and/or shaking the wire loose from the slip-on terminal.
The bumper was the fix for this. This 3rd gen setup was used till 1974. In 1975, a bigger ignition switch was used. New brackets again, 4th gen.
There are so many different brackets used that having mismatched stuff is common changing horn position to who knows where.

Low note (F) horns were factory installed on production models.
1965-E1976 Delco Remy (69000-65).
L1976-1985 Delco Remy (69000-76). This horn was also parts order replacement for 1965-E1976 XLH and XLCH.

High note (A) horns were available individually as replacement parts from 1970-1978.
The high note horns were also available in the “twin horn” kits.
From 1979-1985, they were only available as part of the twin horn kit.
Individual high note horns:
1970-E1976 Delco Remy (69001-70).
L1976-1978 Delco Remy (69016-76). This horn was also parts order replacement for 1965-E1976 XLH and XLCH.

L1976-1978 High Note Horn (69016-76) 2)

The twin horn kits mistakably make one think there are two horns in the package.
Maybe it would have been more understandable it was marketed as an additional horn and parts for creating a dual horn system.
The twin kits only have 1 high note horn in them to add to, or accompany, your stock low note horn creating twin horns.
Also, the low and high horn bodies are mirror images of each other and (constructed opposite).
So the cover for the low horn will not fit on the high horn and vice-versa.
The kits also include bracketry, wiring and fixins to attach and plug both horns into the system.

Twin horn kit (69003-70) was sold for 1970-E1976 XLH and XLCH.
This kit appears in the 1971 parts and accessories catalog as (twin horn kit).
The 1972 P&A catalog lists the kit contents as 1 horn, 2 chrome horn covers, all wiring and fittings.
It has added verbiage “The horn already on the vehicle is used in combination with the one in the kit.
Each has a different tone which blends into a pleasing and powerful blast.

Twin horn kit (69003-76) was sold for L1976-1985 XL, XLCH, XLS and XLX models.
By 1978, 69003-76 kit had replaced -70 kit for 1970-up models.
Below are the contents of this kit from the 1979 Sportster parts catalog.

69003-76 Twin Horn Kit Consists Of 5)
1719W SCREW	No. 8-32 x 5/8 Phillips hd. (6)	3206B SET SCREW, No. 8-32 k 3/16 socket hd. (2)
7015 LOCKWASHER, No. 8 (4)	7041 LOCKWASHER, 5/16 (4)	7608 NUT, No. 8-32 x 1/8 x 11/32 hex. (4)
7748W NUT, 5/16-18 x 17/64 x 1/2 hex. (4)	10006 CABLE STRAP (5)	10026 CLAMP, twin horn (2)
69016-76 HORN, high note	69017-76 COVER, high note horn	69117-65 MOUNTING BRACKET, only (2)
69125-65 RUBBER MOUNT, 5/16-18 threads (2)	69140-76 COVER, low note horn	69144-65 SPACER, horn cover
69149-65 MOUNTING NUT, horn	70016-65A HORN GROUND STRAP (braided) (2)	70318-70 CABLE ASSEMBLY, twin horn
70576-68 WIRE CONNECTOR	70584-69 INSULATOR, flag terminal (2)

The horn mounting brackets are not the actual motor mounts but rather attach to the motor mounts.
Click Here to view information on motor mounts.

This is the bracket the horn directly mounts to. All 1965-1985 Sportster horns are basically hung from a bracket between the cylinders on/from the motor mount.
The horn is hard mounted to it's bracket by a nut on back of the horn.

Horn bracket (69117-65) was used starting with 1967 XLH and XLCH to mount to the horn.
The horn bracket mounts to a support bracket using a rubbermount stud (69125-65).

1967-E1971 XLH/XLCH Horn Mounting Bracket (69117-65) 6)	L1971 XLH/XLCH Horn Mounting Bracket (69117-71) 7)

The rubber mount isolator is a vibration damper for the horn between the 2 mounting bracket involved to mount it.
1965-E1970 rubber mount (69125-65) has 5/16”-18 threads each side.
L1970-1985 rubber mount (62563-65) has 1/4“-20 threads each side.
In L1971, the low seat was available factory installed for the 1st time. ⁸⁾
The low seat required the ignition switch to moved from right side under the hi seat to the left side on top motor mount (where it fouled the horn brackets).
At first new brackets with deeper offsets (that used the old 5/16” rubber mounts) were installed that opened up room for the switch.
There were some issues with interference and the smaller 1/4“ rubber mount and another set of new brackets were furnished to solve that issue.

The 1/4” mount was also used on various 1965-2003 Sportster battery trays and oil tanks.

1965-E1970 Rubber Iso Mount (69125-65) 9)	L1970-up Rubber Iso Mount 10)

The bumper bracket came about in late 1973 due to a rattling horn problem on L71-E73 “low seat” bikes. ¹¹⁾
The bracket part number is (69041-73) and it has the large hole in the center to go over the horn threads on the horn mount bracket.
Also there are 2 smaller holes on each end to pop rubber bumpers (69042-73) in to damper horn movement.
The rubbers keep the horn from flopping around too much, hitting fins and/or shaking the wire loose from slip-on terminal. ¹²⁾
The bumpers got a new part number in 1981 to (11428).

L1973-1985 Sportster Horn Bumper Bracket (69041-73) 13)	1981-1985 Sportster Rubber Horn Bumper (11428) 14)