Bottom Engine Bolt, for use on BSA models: C15.
O.E.M: 40-4075
Note:
I have 3 of these, they came in a box of bits purchased earlier this year. These actually show signs of having welding on the end, probably putting the nut on the end then welded to give the impression of them being viewed as a bolt. I would still use these on my bikes, I have tried them on my bike & it all tightened up as good as the original one.