This is the distinctive kidney shaped rubber shroud that fits over the input wires on the pre-war and immediate post-war Lucas Dynamo - where those dynamos were fitted with a flat bakelite cover, on the Lucas Magdyno units.
Although I cannot be exact on when this type of 'short' dynamo with bakelite cover (as in one of the accompanyning photos) was fitted to Norton singles (and other marques using a similar Lucas Magdyno unit) - I believe it was approximately 1937 - 1951 (although this may vary be a year or so each way).
for these models, the two dynamo wires were screwed into the bakelite cover with a brown fibre plate holding them in, then this rubber shroud fitted over those wires so they were not exposed to the elements. A very handy cover to have as most originals eventually perish
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