I am with Mark on this one. I use the PJ-1 and have plugged all my bikes with that chain oiler system. One less drip on the shop floor. The hole is really pretty small for any effective breathing. In their last year BSA put more effective breathing on the B50 by adding a half inch hose and drilling holes from the primary to the crankcase. I am not sure I agree with mixing engine and primary oil but modern bikes do it but they also have a good filtering system.
These old bikes are a lot of fun to chat about.