B40FT B40 Military - 1965 B40GA WD B40 Australian Army MK 1 - 1967 B40GB WD B40 British Army MK 1 - 1967-70 B40GN WD B40 British Navy MK 1 - 1969 - 70 B40GR WD B40 British Air Force MK 1 - 1968 - 70 B40M B40 Military MK 2 - 1968-69 B40GD WD B40 Danish Army MK 1 - 1967-68 B40GD WD B40 Danish Army MK 1 - 1969-70
For civilian production machines, from 1969 onwards date prefixes were added. But this did not apply to production WD B40 machines - evidence shows only late production factory spare engines used this lettering.
The first letter denotes the month:
A January B February C March D April E May G June H July J August K September N October P November X December
The second letter gives the year from August to July: