MCE, now that brings back memories!! I was there about 5 times, and bought lots of old stock, that they didn't want anymore, they also had the shipment of about 2500 NOS pannier bags from Canada, the green ones for the TRW's, I bought 5 pairs, were quite expensive at the time, about 20 quid a pair, with the Y straps. Mind you, that was in the late 80's...…
Sorry no idea about the inspection cap, but looking at the manufacturing process it does look 70's.
It is a unit engine BSA primary cover plug used for chain adjustment and for clutch adjustments on different models .
I think Triumph used the same one
