Thanks for the pictures, it is now I identified as the viewfinder for the Marconi V3310 colour vidicon camera. see picture 1. I think it was also used on the bigger IO colour camera picture 2.
I do have a V3310 in my collection, or the be more accurate the remains of one. It is quite incomplete. Your viewfinder is a rare Item and the first one I have seen of that type. It is mostly stand alone, accepting a video input, Mains power for the heaters and 250v HT from the camera. I don't think the tube was anything special, Marconi would have selected a standard tube from the domestic TV range.
I wonder what happened to the rest of the camera?
I have found the circuit for it
http://www.tvcameramuseum.org/pdfs/m...1a-circuit.pdf if you want a look, it's quite straightforward.
May I use your pictures of it on the museum site please. contact me directly with full resolution pictures or extra pictures?