Hi Roger,
Many thanks for checking this out and your suggestions. Unfortunately the bulb holder you suggested, won’t fit.
The units I am after are actually the bulb holders. You will see in the photos that the inside of the fitting is threaded and the bulb screws directly into it.
I have checked the fittings on other vehicles of that era that used Jaeger instruments, but it looks like only Tractions produced in Slough, used this particular method of instrument illumination!
Thanks very much for trying, much appreciated!
A bientôt,
Martin N