WD B40s used the Amal type 626 Concentric carburettor;
Amal use to be @ 6,Holford Road, Witton, Birmingham 6. Maybe they stillare?
If you other sources prove inadequate, BMS publishing used to do manuals on carburettors and their applications. They used to be @312, High Street, Dorking Surrey.....maybe they stillare but I think someone else took over Bruce Main-Smith publications!
Cheers
John
JBW