Hi David
I have not seen a washer like that before ??????
the washer needs to butt against the shaft adaptor, the inner drum may or may not sit flush with it
Does the nut fit in the recess? but then what happens if you use the lockwasher???
All the washers I have fitted are plain both sides about 5mm thick, theres a spigot on the nut to centralise the washer
John