The BSA with frame number WM20.41890 has left the Birmingham factory on 21/8/1941, as one of the last bikes of military contract C/7287. This was a contract for 17.000 bikes (frame numbers WM20.25001 - WM20.42000). The census number would have been C4358482.
Don't know where you live Jordan, but this bike is not in the Australian AWM ledgers (although several bikes from this contract were transferred to the Australian Forces, inclusive WM20.41891). It is also not in the post war KeyCards, which means that it didn't serve in the post war British Army.
These are the closest numbers that I can find in my pictures. The second picture shows one of the bikes that had been transferred to Australia (with the "old" British census number still on the tank, but with an AIF number added to the front numberplate).
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