On military Nortons a variety of metals surface protection was used. The most obvious is of course the paint used which most likely was a cellulose based version as commonly used in those days. The top coat was initially applied without any primer but at a certain time parts were covered with a red oxide primer before the application of the top coat. It is not clear if the topcoat was a different chemical when used in combination with the primer.

During the pre-war years bare metal surface protection of handlebar controls, foot rest hangers, kick starter, rear brake foot pedal, gear change lever, exhaust pipe and silence, wheel hub dust caps, oil and petrol caps and lines, nuts and bolts was a dull chrome finish, contrary to the bright chrome used on civilian machines for those similar parts (See also original specification page).

a 1936 test machine showing the use of matt chrome and blackened parts.

When hostilities started, chrome became a strategic metal to be used for all sorts of military equipment to increase hardness and corrosion resistance and was as such replaced by cadmium for basically all of the before mentioned dull chrome parts.
For reasons unknown it appears that from late 1943 onwards exhausts and silencers were painted black (or chemically oxidised?). A possible reason might be to conserve cadmium for more important parts in e.g. corrosion protection on aeroplanes and naval equipment.

A few parts were either blackened through chemical oxidation or oil blackened, actual process not known. Example of this were the carburetter inlet stub, bolt fixing the Horn bracket to the horn clamp around the front frame tube, nuts on the gearbox cover and possibly the kickstarter spring cover. Later in the war various other parts were also blackenend like for instance the handlebar clamp bolts which were initially painted.

Cad plated exhaust clamps contrasting
with the black painted? exhaust on a 1945 built machine.