Good work getting your Duco 486 electric tail lamp identified. I have a lamp similar to yours at home but it is missing the red reflector. I always wondered what it was as there are no markings. I can send a photo when I return home next month.
I'm not meaning to hijack your topic but to add information about another rare lamp to it.
The Lucas ST20 "Stop" Tail Lamp is shown on the same Brown Brothers catalogue page.
In 1939 Ariel offered a modified rear number plate, 5648-39, to fit the Lucas ST20 "Stop" Tail Lamp.
This modified rear number plate was available for one year only.
The clip shown below is from 1939 Ariel Parts Book, Page 44
Shown below is a photo of this number plate from September 2007, page 14, Cheval.
More detailed information.http://www.arielownersmcc.co.uk/members ... cation.pdfhttp://www.arielownersmcc.co.uk/members ... ype_20.pdf