G'Day.
It may have been part of a Service Exchange Unit. Page 37 of the 49-53 parts Catalogue has a note that a damaged crankcase half could be replaced with a new casting, and by all accounts factory matched to the existing used good half. But from experience, it is the timing side that usually survives, a rod through the drive side case being the usual malady.
So that's a possibility, but halfway round the world hardly likely, however the stamp font used looks BSA factory style. Maybe they just kept making spasmodic batches?
Date on the Drive Side Case is from a time of change and case may have the early crankshaft oil slinger or the later proper oilseal. The only difference is the size of the hole, early case can be opened up to take a seal. If the drive side outer race is still in place, and no seal fitted, the slinger should be found still in place between race and case. I mentioned this difference in design with an earlier post.
Cheers
Swarfy.