Chris,
On the Aero the mainshaft pokes through the bearing, end wall, and then terminates with a thread for the kickstart ratchet (nut).
1936 Aero 600 4-speed:
My 1934 OW1 (ohv) is the same way; same gears in a different sheel turned 90 degrees and mounted up under the saddle).
There were some minor developmental variations to the 4-speed since it was introduced c1932. Perhaps the earlier version extended the shaft further? I would suspect that the ratchet pinion was then on the end of the layshaft, held off by a light spring, and capped by a nut. The earlier 3-speed had a kick start ratchet like that. The 4-speed as on my OW1 and Aero have a kickstart quadrant that is in constant mesh with the ratchet pinion. So the pinion is not on (and rotating with) the mainshaft, but floats co-axially on a idler post attached to the gearbox cover. The nut for securing this post can be seen externally on the end cover.
I do have a 4-speed trans for my 1932 Greyhound project, but not sure if the cover is loose that I could readily have a look. I'll have to dig it out first!
-Doug