Ladies and Gentlemen assembled,
I believe I may have solved the mystery.
The electrical diagram for the 1935 Alvis Speed 20 clearly shows a single three light unit for Stop (Top Left), Tail (Bottom Centre) and Reversing light (Top Right). The draughtsman has thoughtfully represented the unit as a triangle containing three circles and a key.
There is a makers mark, in the centre, with something like "Monarch? King? of? the? (illegible) Road" around the outside. There is also something like a prancing horse (ala Ferrari) and something like a torch, clenched fist or bunch of thistles at the top of the badge.
The picture probably shows the unit upside down, and it might have been hung from the offside bumper bar attaching point.
My conclusion is that this company supplied these units to the Alvis Works, in the period!
Many thanks to all who provided ideas.
Edited by 275 GTB-4, 25 February 2013 - 05:15.