They are indeed plated brass and with a thick washer under. Initially dull-chrome but presumably cadmium plated later.

They're not listed in the Norton Spare Parts lists as they are Lucas components. LU/112561 (screw) and LU/137141 (washer).

Would brass have been used to facilite earthing of the shell or is it simply that it nips-up with less spanner pressure ?

Merci,

Tout est bien!