I can't give you chapter and book (read: complete part #s) on this, but I had my learning curve a few years ago when I bought some from K&K Mfg. My experience with the 107-numbered donuts was the same as yours; must be happening to someone somewhere all the time. My phone queries to K&K resulted in their claim that, for a decade or two at least, they had not stocked or sold any other part for the exhaust application we're discussing here and were pretty certain there never had been any other part for it, ever, period. But there is a 113-numbered donut, and when I had one in my hand it was easy to realize how silly it is to think that a part which is numbered for a follow-on design would be original to a car manufactured in the preceding decade -- especially a German car. The one that's appropriate for our cars is thinner and of a larger diameter. Fitting it is not exactly for sissies either, but is entirely doable, especially with a little leverage of the type you've already employed.
Incidentally, JA17 made a slick & simple tool for this application years ago from a long-shafted screwdriver. Joe, would you post something to guide us in making our own copies, please?
I can't access the EPC but can't imagine that the full 113-prefixed part number for the proper rubber donuts is not plainly given there. My best source for it is on (underneath) the car now, as the correct part carries the number plainly on it, at least when new. So no, your're 'not insane', Eric; ya gotta trust me on this. It's like there are two different worlds for this tightly-circumscribed little subject. It's like ..... like in the movies, man: as soon as you make contact with the world in which the 113 donut exists or is recognized, then everything will seem normal again.
Actually, I mis-spoke: here it is in an old paper parts book: 113 492 00 82. Same digits as yours except the first group moves the application back to our Pagodas from the follow-on Panzerwagen models -- 107 -- that the donuts you have are meant to fit. [It's standardized that the first grouping gives the designation of the model on which the part first appeared. The other digits are decodable too.] In this manual (#10202) it gives the 113 part's application as being for the rear hanger site and the 112 492 02 82 pair as being for the more forward, or mid-car site, the reverse from how Jordan's source has it. And FWIW, the 112 part is illustrated as a donut similar to the 107s, while the 113 at the rear is shown as a classic 'racetrack' ellipse. Further confusing is the fact that I believe I only had and installed round ones ..... and they definitely had 113 part numbers. So now there are three sizes in play. Isn't this all very helpful?
Well, the primary takeaway is to toss the 107s; it gets a lot easier after that. Man! I remember that I did not care for the first parts of this episode when I went through it on my car.
