I stand to be corrected (I've just come back from the Mid Glos Annual Dinner and am suitably oiled!) There are 2 sizes of thread used on the kickstart end of the mainshaft. Series A boxes and possibly some early C's used a smaller thread (1/2" x 20 tpi) where as later boxes used 5/8" x 20 tpi.
Brian Werrett
Brian Werrett