This is the shim fitted behind the selector fork and rear mount plate on both Upright and Laydown gearboxs from 1930's to late 1950's.
I am told by someone in the know that it is very important to have this shim fitted to ensure a smooth gear selection. What I do know is that in the majority of gearbox's I have stripped down have this shim missing! - so it is worth checking yours to check if fitted.
The pattern for ours was taken from a genuine and unmolested upright 1940's Manx Norton gearbox. It was fitted to all types of Norton upright gearbox (i.e. 1931'ish onwards) and laydown gearbox, up until the introduction of the AMC style gearbox.
In the first photograph, the mechanism on the left is a typical (1930's-40's) upright gearbox selector mechanism with the shim fitted, while the mechanism on the right is from a 1950's laydown gearbox with shim fitted.
See also item 0852 for similar (non-standard?) front pawl shim