Giel, here is the entry. Sorry that it doesn't help much !
Saying that the early BSA ledgers are a complete chaos would be the understatement of the year... I will try to explain. Before the war (KM20 production), bikes didn't have "matching numbers". Engine numbers were listed in the engine ledgers, and frame numbers were listed in the frame ledgers:
Engine ledgers:
- The KM20 engine numbers sequence was listed in the KM20 engine ledger,
- The KM21 engine numbers sequence was listed in the KM21 engine ledger,
- There were also engine ledgers for the KM22, KM23, ...
Against every engine number we can find the frame number in which the engine was fitted. As all these engines used one single frame number sequence, I think it is easy to understand that there was no logic at all in the relation between frame number and engine number.
Frame ledger:
All the KM series bikes used "the same" frame, so all the frame numbers were listed (in sequence) in one single frame ledger.
Against every frame number we can find the engine number of the engine that was fitted into this frame. This was of course a mixture of KM20, KM21, ... engines. Once again, I think it is easy to understand that there was no logic at all in the relation between frame number and engine number.
After the invasion of Poland on September 1st 1939, Britain declared war on Germany on September 3rd 1939. A couple of days later the "war production" started. The KM20 now became a WM20 (W for War production). The first WM20 was frame number WM20.111, and the first batch (frame numbers WM20.111 - WM20.251) all went to the Ministry of Supply (contract number unknown). Up until frame number WM20.243 there were still no "matching numbers" (engine numbers were chosen "at random" from the WM20.1700 - WM20.1900 range (*). This was a continuation of the KM20 engine numbers sequence).
From 13/09/1939 onwards (frame number WM20.244 to be precise), BSA started using "matching numbers" for all the military production bikes (contracts for the Ministry of Supply, Dutch Army, Irish Army).(**) But the civilian bikes that were still being produced in between the military contracts still had non-matching numbers (with exceptions). These were no longer numbers in the WM20.1700 - WM20.1900 range, but for the civilian bikes they were now using up the pre WM20.244 engine numbers. (***)
Back to our engine number WM20.864. When we take a look at the engine ledger, the WM20.864 line is empty. We could assume that they forgot to add the "ditto" marks under WM20.861. In that case the numbers WM20.861 up until WM20.900 would all be military bikes. If this would be the case, we should also find the numbers WM20.861 up until WM20.900 in the frame ledger, with matching engine numbers (**).
Now let's have a look at the frame ledger. Here we can see that frame numbers WM20.861 up until WM20.900 are all civilian production (***). Two remarks:
- WM20.861 is military production according to the engine ledger, so should have had matching numbers. But frame number WM20.861 is civilian production according to the frame ledger. Obviously there's an error somewhere, I think the engine ledger is wrong.
- The WM20.864 entry in the frame ledger is empty... Did they forget to fill in the data, or was frame number WM20.864 never produced? But as it was most probably a civilian bike, the engine wouldn't have been WM20.864 anyway...
So let's assume that this engine comes from a civilian motorcycle. The early civilian bikes had engine numbers in the WM20.1700 - WM20.1900 range (*), and the later civilian production bikes had pre WM20.244 engine numbers. (***) So WM20.864 doesn't fit in either of these sequences...
I've also trawled through the WM20 frame ledger in search for engine number WM20.864, but I couldn't find it... Conclusion: this engine wasn't fitted to a frame (civilian or military). Rest the possibility that this was a spare engine, presumably from an early military contract...
Hope this helps... 😊
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