Hi tumbleweed , is your bike a plunger framed A10? . The plungers had a semi unit construction with the gearbox bolted directly to the back of the engine and the innerprimary chaincase attached to the crankcase the swinging arm version has the screw holes and 2 bolt holes to fit a detachable chaincase. Sounds like a plunger be careful fitting just one other crankcase ,they were fitted has a pair so you may find things just slightly out when you put the other case on and can get leaks and even worse ,check carefully if you go down that route,regards bobh