Ah, so the shim provides tightness, not positioning, as its primary function? In theory, any ring that
fits would work?
No no and no. Any ring that fits
as below would work.
Think of a tripod. What would happen if you removed one or two legs? It falls down.
Somebody somewhere started calling these shims which is a misnomer. They are
adapters. That is their job. Not to shim the bulb for any reason but to allow an H4 bulb to be used in an 1100 starting with 60/55W and higher halogens. They predate LEDs.
Imagine the hole for the bulb in an ST1100 headlight assembly (reflector) is the diameter of the
outside of the ring above (excluding tabs). If not for
all three tabs - the ring and anything attached to the inside of it - would fall into that hole. The tabs provide support (the main function) and orientation of the bulb.
Now imagine the hole in the ST1300 headlight reflector is the diameter of the
inside of the adapter above. There's no way the adapter with two or even all of its tabs modded would fall in. The 12 o'clock tab provides orientation but is not needed for support as the 1300's reflector supports the bulb by the rim and not by the tabs. So the adapter is superfluous in a 1300.
That's the difference between our 1100 and 1300 assemblies where the bulb fitment is concerned. The former needs all three tabs and the latter actually doesn't need any (but the 12 o'clock makes orientation simple).