@Craig: If you have an original WM21 from 1940 it is likely that your BSA was originally destined for the Dutch Army. After the occupation of the Netherlands the remaining WM21's were bought by the War Department. The Dutch Army ordered the WM21 with pillion footrests but without the BSA pillion seat. The Dutch Army fitted German Gefrag pillionseats on the WM21's, these were far better than the BSA pillionseats.
The WM21's which were bought by the War Department didn't have a pillionseat fitted, only the standard early rear carrier.
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.