Did a little more digging, check this out:
So you use those two parts if the groove on your first/reverse gear for the ring is 1/8", otherwise discard. As a side note the retainer is .062" if it is put into an .125 groove,one could see how it could possibly pop out, most R&R parts kits have the wider retainer to make it a nice snug fit, eliminating the need for the friction material and special retainer. GM must of gotten a few first/reverse gears with a different spec on the machining on the retainer groove. Who knows it is ancient history and all the guys fixing them are far and few.
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Last edited by sstock; 11/03/2019 2:00 AM.
1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration