Remember when I said "if they used the same design as the 3-speed". Well they did, and they did not. I pulled the sprocket off of my 1936 Aero gearbox and the seal is not the same as the 3-speed. The housing has no external access to the seal, as you can see in this picture. The aluminum comes right down to the journal diameter.
Which did not match you photo at all. So I looked at another gearbox, this for a 1932 Greyhound 4-speed. It has a design like the 3-speed and the section illustration that I previously posted.
Just to the rear of the keys you can see the spring washer. The spring washer and the backing washer behind it are shown below. The leather gland was missing.
With this style, the back of the sprocket has a more pronounced spigot that projects into the end recess of the gear case and bear against the spring.
The 1936 sprocket only has a slight hub. The 1936 gearbox for the Aero has a P prefix, the 1932 gearbox for the Greyhound has a GB prefix. It looks like you have this earlier style gearbox?
The spring has a cross-section of 0.062 thick by 0.093 radial, 1.531 outer diameter, and a free height of 0.200 (all dimensions in inches). The backing washer is formed by rolling a lip. The material is 0.036 thick with a inner flange rolled up to make a overall width of 0.088; the outer diameter is 1.739 and the inner diameter is 1.293 (inches). The lip faces outward, towards the spring.