This is a copy of the oil retaining shim that was shown as fitted between the main gearbox bearing and gearbox shell on pre-war Norton spare parts list - part no 9666, and just like our Item 0853 (which was designed to be fitted inside the gearbox shell between bearing and sleevegear) - was designed to reduce oil loss from the gearbox.
However, it is worth noting that this shim was never shown fitted on post-war spare parts catalogs - instead just relying on the larger diameter shim (Item 0853) fitted between the sleevegear and main bearing for oil retention. Indeed, most people (myself included for many years!) have never stripped down a Norton gearbox with a shim fitted where this was originally intended to be fitted - between the main bearing inner race and the gearbox shell.
Presumably, Norton found them superfluous and decided not to continue with them post-war.
If you wish to fit this bearing shim - then this shim should be correct as it was taken from an original Norton shim of the same dimensions (I say should . .. because it is extremely difficult to be absolutely sure the gearbox this was taken from had not had a main bearing change in later years . . . but looks correct).
However, the other useful purpose for this shim - it can also be used as an alternative for Item 0853, if you do not like the idea of fully shielding the gearbox main bearing from the gearbox oil - which the original sleevegear/bearing shim must have done to a certain extent. In addition, this size shim if used for that purpose should not wear around the sleevegear teeth area like original shims of the larger size were prone to do.
In the past I have stripped later (laydown type) Norton gearbox's from the mid 1950's and have found they have been fitted with Sleevegear/bearing shims (i.e. A2/417, Item 0853) of the same size as this shim, rather than the wider shim shown in the 1950's spare parts catalog - so maybe that was a later change, or owners have fitted them themselves as a later change. As I say in the detailed Item 0853 description - I will leave the choice with you which you wish to fit - both shims are the same thickness