[Cal Metal]

Have to agree with Road Racer. It is more likely either the 1-2 synchronizer unit and/or second gear or both. Given the inexpensive nature of the parts, I would replace both of them as a set. David Kee has all the parts, including the shifter rails and brand new detents and springs. He is the only one on the planet who has the rails and detents that are brand new. Do it right and you won't have to tear it down again.

The typical fix are the blocker rings but they have NOTHING to do with retaining engagement between the sychro unit and the gear. The problem you describe was very typical on the old Borg Warner transmissions. After awhile, they give up the ghost and both pieces needed to be replaced.

Regarding the set screw and spring, some came with the screw and some did not. You can use the correct non screw spring in a case that has provisions for the set screw. The case you have is the best, imho, as the lid holds down the spring.