Hi Bastiaan. Thank you for that info about the Dutch contract frames. I was unsure if my subframe is correct. (rear footrests; no field stand lug).
My bike was part of the 1940 C7370 contract, which didn't make it to the Netherlands. Unfortunately I haven't been able to find any record of where it was eventually deployed. I'm trying to keep it faithful to early 1940 spec, but will fit a British-type pillion seat (one of Jan's) to suit the rear pegs.
Your rear frame should have rear stand lugs, no field stand lug, 1 sidecar attachment on the left side and a fitment for a short propstand on the left side.
See the photo below of the type of rear frame which should be fitted to your WM21.