Ok, I have at least a dozen or so Sm420 apart this last year from 48-67 models, never seen those parts inside a trans. However, it could be part of a TSB for transmission issues (post 1957)that have affected the second gear synchro retainer. I have seen two transmissions that have had their retainers dislodged after disassembly and inspection. My theory is the retainer can rotate when the synchro ring is pressed into the first gear friction material during a downshift, perhaps dislodging the retainer on extremely rare instances, extra wide retainer you show and the rubber friction material would stop ring rotation. I say use those parts, the rubber would probably just be laid into the retainer groove on the first/reverse gear then snap the retainer in after placing the ring in
.
Steve

Last edited by sstock; 11/03/2019 1:41 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