Wonderful research and analysis, Jan. The letter confirms that by 1971, BSA had clearly lost track of the interface 'twixt arse and elbow, as far as wartime production was concerned. We can't therefore blame the closure of Small Heath and the removal of records at that time.
Someone at some point decided not to retain, but why then not dump the earlier stuff too ? Were there still a few overseas customers in the earlier records ?
We have a similar but more accute problem with the Norton records. Folklore has all the Bracebridge Street ledgers being dragged out of a skip at Andover. Considering AMC's attitude to Norton, it's a wonder that they even got that far...but as per BSA, the pre-war records were preserved more or less in their entirety, although with discrepancies in engine and frame book retention.
It may be worth bearing in mind that factory ledgers show despatch rather than production..and the Despatch Department must have been where the ledgers were held. These books are not a summary of the Tally Cards, but of when the bikes were put on the dropsides or the goods wagons. Was the bomb damage in November to the production facilities, or the Despatch Department ? I suspect production, or they would have found another way to despatch. It may just have been one or two components that held things up...
My neighbour worked at BSA around this time, as has spoke about one bomb that was dropped that took out a few floors, and killed a few workers.
He was called up and served in the RAF Regiment on AA guns, still around at 101 and a 1/2.