Thank you for your help, Jan! As i think that it might be interesting for others to know, here is what he found out:
"Frame number 130xx was despatched from the factory in Redditch on 12/12/1942 to the War Office in Wilford, as part of military contract C/13869 (3000 motorcycles with an Albion gearbox). This was the sister contract of contract C/13870, which was the contract for 3000 motorcycles with a Burman gearbox. It would have had census number C4942793 on both sides of the petrol tank.
Although the “duplicated frame number” (stamped just below the cylinder base) is also 130xx, the engine in your bike is not the original engine. The number under the cylinder base has been restamped, if you look closely you will see a flat surface where the original number has been filed out. Some of the military bikes were reconditioned by the factory after that war, and engines and frames often got mixed up. The factory had a habit of restamping the duplicated frame number in a specific font. Your engine has this font, and in the factory ledgers I did indeed find out that 130xx has been factory rebuilt. It was despatched from the factory (for the second time) on 13/04/1948 to Cruickshank in Aberdeen. (As i may add: In my german vehicle papers ("Zulassungsbescheinigung Teil 1"), the "Fahrzeugschein", it says that the date of first registration ("Datum der Erstzulassung des Fahrzeugs") is 31.05.1948.)
When I compare your “engine number” (20505) with other entries in my Register, I can see that it must have come from a motorcycle with a frame number around 9000 (which would indicate May 1944). This would have been a contract C/12425 number, which is confirmed by the contract number stamp on the engine: C/12425. The “acceptance marking” for this contract is ^M426. The “4” on your engine is barely visible though.
The gearbox with prefix HDM is definitely post war. A war-time gearbox should have a HH* prefix."
With the help of Jans documents i found out that my WD/CO has a "type 1 frame" and a "type 1 1/2 fork with steering damper and check springs".