These HMV and EMI ribbon microphones show up quite often for repair. The models are identical, just branded differently, and very often they come complete with wooden box, which survives decades in a damp shed or attic better than cardboard.
It is rare to find any paperwork with these but I was lucky enough to come across an HMV 2350 complete with brochure and manual
The manual confirms that they were designed to drive a 20 ohm line. I have found that most of these mics measure around 15 ohms and have really low output. A suitable matching transformer makes a big difference to the usability of these microphones.
PS, some of you might recognise this mic from Marvel’s Captain America movie – it was used by Howard Stark in the scene where Steve gets his treatment!