CuzzFuzz, After a bit of sleep to help clear the cobwebs from the gray matter in my head, I realized the problem you reported having with the replacement mounts is they do not have the slope marked in the circle of the original mount in the below snapshot of your original pic. Sorry for the slow up-take on the understanding.
I looked through a number of our regular vendors web ads and although they show new mounts with that slope pictured, the reproduction products themselves all seem to be the "flat" ones to the right in that pic.
I think the answer may be, other than sending your originals out for revulcanizing, to see about snagging some of the several NOS/NORS rear mounts currently being offered on ePay. These all have pictured mounts showing the sloped edge you need. Here is a sampling that I found this morning:
A pair of rear mountsA single NORS rear mountA single NOS rear mountAnother NORS rear mount with 8 available for purchaseI am sure that there are others, but these looked like the best opportunities to find what you need. Hope this info is helpful.
