Thanks for the suggestions everyone! Unexpectedly, the order the shim was on has shipped! I am suspicious, however. I think they may have let the 90 day time frame they originally quoted for the "national backorder" run out and then crossed that shim off the order and shipped the rest (some side shims). I don't know that though. I will have to see what shows up. In the meantime, I was able to remove the rear pinion bearing today and free the existing shim for measuring and test filing:
Shaft collar side and gear side, respectively:
The OD is 53-54 mm and the ID is 35 mm. With regard to filing, I ran a fine single cut file across the surface of the left picture very lightly and it immediately started removing the dark coloring from the outer edge of the shim. I want to say that where the "collar" contacts... or sits against the shim, it is now lower (compressed?) than the outer edges of the shim. I did not want to, nor did I file more than a few light strokes on the surface of the shim.
I also note some very light chaffing on the surface of the shim. I suspect that would be hard to achieve on a "hard" shim. I think this thing is actually pretty soft, for whatever that is worth.