I currently have 8 rear backing plates. A mixture of Bendix 317558 317559 and KH 71086 71085
Only one of them has small number, and a letter, stamped near the adjusting hole.
On a side note, i have a Bendix 323369 that appears identical to a 317558. I tried but cantdetermine its original and proper application. It functioned on my 69 Gt500 for 10 years but not its correct home. I once owned a torino with a 9 inch so maybe i cross pollinated it back when.