6 speed manual gearbox rebuild

[Superdave]

2015 Focus with 1.0 EB and Getrag 6 speed manual. Jumping out of reverse.  Installed a used transmission and tore apart the old. Found broken off ears of a small ring between the reverse synchronizer and the “sprag”.  Uncertain of the parts names. Anyway, has anyone ever rebuilt this transmission? I read it may be called a IB6 or 6MMT250. Gears, bearings, shift forks appear sufficiently robust.  It’s always a cheap, weak part that breaks.  

[Hearsepilot]

I'd hazard a guess at Baulk rings or syncromesh