Thank you for sharing that Bastiaan, the front of the cover is interesting it says "Model 1939 M21", there are few references in the BSA KM ledgers for "M39-20" which may have a connection?
In total 10x BSA M20 were delivered to the Dutch Army, 7 of them were with handgear change and in 'army green' as mentioned in the ledgers. I expect that the other 3 were also with handgear change and in army green but it's not specified in the ledgers.
4 of those M20's were delivered to the Corps of Motorcycle Service (Korps Motor Dienst), these all had handgear change
1 of those was delivered to a unit/department which was abbreviated as 'A.M.A.' I have no clue yet which unit/department this was