Shouldered 3/8" Whit Hex gearbox belt pulley nut to fit Douglas 2 3/4hp 2-speed (and 3 speed?) gearbox's - in stainless steel with linished hex.
This shouldered nut is CNC manufactured in stainless steel, with imperial 3/8" Whit hex spanner fitting and correct 7/16 x 20tpi whit form thread (not UNF). The overall length of the nut is 0.750", but the thread is only 0.500" of that depth (this being the length of thread on the gearbox shaft) - this type of shouldered nut meaning it is easier to get a ring spanner to tighten the nut, when used in the recessed type of belt pulley often fitted to this model.
In truth, I cannot guarantee that the original Douglas nuts were of identical profile, as the early Douglas spare parts catalogs show various numbers and do not have any clear picture of what they look like - but I manufactured this nut profile to be a good fit in my own 1919 Douglas belt pulley (as per photos), and it's length allows me to finger tighten the nut easily, then get a socket or ring spanner easily to it.
I think it also looks nice and period correct, if not using the tin cover (which was sometimes fitted over pulleys). It is CNC manufactured in stainless steel , but we linish the faces of the hex as well, therefore it is a very pretty nut.
The thread of the nut is 7/16 x 20tpi, and this is certainly the thread fitted to the (various) 2 speed gearbox's I have - however, this nut may also fit other Douglas 2 and 3 speed gearbox's of similar era - please check the OD and thread of your own gearbox before ordering. Likewise, I see no reason why this nut cannot also be used with the other commonly fitted belt pulley of similar era - that being the type without recess and no holes drilled (I have another gearbox/pulley of that type for a second Douglas project I have)