Weihrauch apparently started using shims to fine-tune the fit between the breech jaws and breech block in the early 1970's. It's not unusual to find guns from those days with a shim on only one side.
The HW 35 shims are larger in outer diameter than the HW 55 ones (because its receiver tube is larger in diameter, and thus the breech jaws are taller). You could probably use them in the 55, but they will be visible above and below the breech jaws. The HW 55 shims are the same as used in the HW 30 (R7), HW 50 (old one), and other guns with the same smaller receiver diameter.
In an older gun without shims, you may need to spread the breech jaws slightly to get the shims in there. A quick and dirty way to do that is to put a chunk of metal between the jaws that is slightly narrower than the gap, then reverse the breech pivot bolt and use it to push against the metal block. Go slow and BE CAREFUL, easy to over-bend the jaws if you get over-zealous.
If your gun doesn't show evidence of wear on the breech block from the jaws, then there is no need to fix what isn't broken!
I usually add them to older guns but that's just my preference. It cuts down on additional wear, makes cocking smoother, and renders it easier to fine-tune the breech tension adjustment. And of course you can easily remove them to return to OEM condition.
I may not have been clear before--HW 55 uses exactly same shims as R7, so they are cheap'n'easy enuff to find these days.
This message has been edited by MDriskill on Sep 4, 2012 11:34 AM