Steel rivet that holds the synchro hub / ring / cone to the gear on 2nd & 3rd gears on Feltham cars, if you are having difficulty with gears baulking or are stripping the gearbox, you may find the rings are loose on the gears, to check this remove the gearbox top, put a screwdriver between the steel ring / cone & the gear and gently twist the screwdriver to see if there is any play.

Countersunk rivet 1.25" x 1/8"

16 to a gearbox, we reckon to use 20 as there are always a few failed rivet sessions