Adrian..You must remember that when these bikes were new, they were relied upon to take their owner reliably as and when required. So, if you broke a bolt on a Sunday morning, anything to hand was used or adapted, as then the whole country came to a halt on Sundays.
BSA were unusual in their use of the fine cycle thread, which was ideal for the application and suited them from their cycle manufacturing heritage. Spare cycle thread nuts and bolts were rare in the average domestic setting. However, most blokes of the time had a tin of nuts and bolts, most often coarse Whitworth, which could usually be relied upon to fit nothing. So Whitworth bolts were forced down UNC threads, sometimes the sizes matched (almost) well good enough. BSF and cycle? Almost but not quite. So, you get the picture, that's how we get these mechanical travesties.
Someone has decided that the easiest way around his problem was to use bolts, so that's what happened, and sod the consequences for the next lucky owner.
Your head should have cycle thread on the inlet tract. If it has been re-tapped metric, M8 will be the thread to try now, but you may find the standard M8 bolt head fouls on the carb body.
Swarfy.
Additional. Parts book lists 02-129 as the official bolt. Once more musky's very useful chart helps us out, identifying it as 5/16" CEI Thread, 3/4" bolt, but with a small hex tall head to aid installation. This head is the same hex as the thin walled gearbox cover nuts.
