The circular gearbox sprocket cover plate / oil seal holder in the back of the primary chaincase has a small protrusion on the external face. Does the cover have to be fitted in a specific orientation so that the protrusion performs whatever its purpose is - perhaps related to the chainguard?
Also - I have fitted the oil seal into the cover with the open side facing into the chaincase. But in the Rupert Ratio book pg 118 it looks as though the seal is the other way around (but that might be a trick of the light).
Open side faces in towards the oil.
Open side of seal faces into the chaincase.
The best reason I've found for the protrusion on the back of the sprocket cover is to help remove it. If the cover is fitted with the protrusion at 12 o'clock it's easy to put a tyre lever or similar between the sprocket and the protrusion and apply pressure to the protrusion to push the sprocket cover out.
But I've never seen anything written down by BSA to that effect.