Old Joe Lucas must have thought it a good idea...as a parking light. Alas it was a short lived styling exercise being pretty dangerous to ride a bike on sidelights as a good few folks did.....".to save the battery" Yup, ignorance of the charging system goes back a long way. Some fools still even drive on sidelights, despite campaigns to use dipped headlights. So back then it was quickly dropped.
Lucas sold the same item to Triumph, and it is also still available new. Basic lens and lampholder feature as the guts of the sidelamp fitting for a Daimlet Dart!!!, the sporting 1950's aristos' vehicle of choice with a current price to match. Hurry while stocks last at around £75. I won't be in the rush. Prince of Darkness indeed. Sod that.
Swarfy