A bit of background.

Here's what I know about the engine's provenance: it was bought and sold at least a couple of times, the last of which was at one of the regular classic bike autojumbles where blokes go rummaging in big grimy piles of dirty, rusty, sticky, oily metal sitting in the open air, in the baking sun or under the pouring rain.
As I mentioned before, I have no idea why the engine became separated from the rest of the motorcycle in the first place, but judging by the way it seems to have been severed, I'd say it was probably done at a breaker's yard? Or perhaps the bike was in an accident and they salvaged the engine? Maybe they had a 650 engine and swapped it out?
I doubt I'll ever find out, though I might get in touch with the owners club and see if they can trace any history based on the engine numbers. Then again there are only slim chances of that yielding any results seeing as, from what I gather, BSA engine numbers are more of a rough guide than a precise indication.
What I know for sure is that many people have had this engine, but few of them have actually used it. It just sort of got passed around until it reached me - and even then, it's been sitting in a workshop half dismanteled for a good while now.