Hi Grzegorz,
Number WM20.5874 is a contract C/5110 bike. Theoretically the census number would have been C3929292, but from the Key Cards we know that there were a few anomalies with the census numbers from this contract and in reality your bike had number C3929293. It was despatched from the factory on September 28th 1939. Please note that the census number "style" (C on top of the number) differs from your later bike (C in front of the number)...
As can be seen in the pictures below ("sister" bikes of WM20.5874), a lot of the C/5110 motorcycles were sent to France with the BEF. Makes you wonder what the story of your bike may have been: BEF bike captured by the Germans and sent to the eastern Front, and then back to Poland...?
Jan
email (option): wd.register@gmail.com