Hi James
The spots you see are a defect in the manufacture/design. Some of the early CRTs have a defect with the tube faceplate bonding. This leads to spots on between the tube and the front glass. I have a number of cameras and spare tubes in the collection that are affected, some of them very badly. Very early tubes used in the MkV cameras seem less affected and very late tubes are unaffected (yet). The Marconi MkIX is affected as well and cameras from other manufactures too. It is a real problem.
The worst case I have seen
http://www.tvcameramuseum.org/link/1...k103large2.htm
7" viewfinders without the bonded on face plate had a glass or perspex window in front of the tube with a clamp band around the tube for fixing. In the bonded tube the mounting flange is part of the fixing.
I have sometimes thought of picking open the seals at the edge and cleaning it all out, but I can't recommend this as the tube might go bang! which is to be avoided.
The Marconi MkVIII tube number is PMT69 I am sure there is a standard equiv number in the MW xxx range.
I would be interested to know how you get on.
Brian S