Silly me. I expected a simple answer.
I can imagine that the first engines were not fitted, given the fact that BSA knew there were lube problems. My engine, at 1200 miles had the 2 exhaust valve guides removed which says a lot.
And most of the 'innards' were gone. So if the first 120 were never fitted, and numbering starts at 101, is my bike the 59th complete bike out of the factory?
I can post a photo of the frame # but it is clearly 280 to match the engine. And it has the 'pogo stick' center stand that was (wisely) discontinued after # 300.
Well, maybe mysteries are more fun than answers.
And, thanks again for everyone, for talking through the rebuilt.