Remember the cork? It was filling the timing hole in the crankcase after the plug dropped out.
Now, it vaguely occurred to me that it was strange that a bolt could back itself out like that, but apparently it happens, so I just ordered a plug for a '52.
It didn't fit, which should have been my second clue.
I then ordered an oversize plug, not even thinking to inspect the timing hole itself. And then when I came to fit it, I realised that the hole's threads are knackered. I'm guessing someone just forced in a bigger plug at some time without tapping the hole properly. Or something.
After a bit of online research I realised that this is fairly common, isn't an easy fix and that some people do run corks in their ruined timing holes... permanently.
Hmmm. Any ideas? I really don't want to be splitting cases on a great running bike...