The model number ain't gonna mean a lot to anyone as this is a very
old piece of kit (1950s) that I keep mainly out of a sense of
reverence for the past. :-) It's an old ex-RAF AVO. I have posted a
picture of the socket to a.b.s.e under this same thread title...
BTW, there's a 5p coin shown for scale, but since that won't mean much
to anyone outside Britain, the outer of the socket is approx. 1" in
diameter (which won't mean much to anyone in europe but it serves them
right for adopting the metric system.:-))
======================
Since Britain is part of Europe you probably mean 'anyone in continental
europe'

Just for your info ,the metric system was introduced /imposed by Napoleon
(eg more than 200 years ago)
For once he did the right thing .

Frank GM0CSZ / KN6WH