These overhead valve machines (with iron cylinder and iron head) were made between 1946 and 1962. Rigid frames, plunger and swinging arm.
Many parts interchange with 2 cylinder A7 and A10 and M20 and M21 sidevalves and Gold Stars. If you can't find it here, type an original part number in the finder...
The swinging arm models use the STD gearbox
The rigid frame bikes share the gearbox with M20