Thanks Guys,
All interesting stuff. Terry, Jim, I don't think it is part of the U/C system, I have full drawings and operating instructions for that and it doesn't seem to fit anywhere. Also, the later aircraft fitted with hand pump U/C don't have that fitting.
Mike got onto me today to let me know that the aircraft in the German assessment is K9964.
IT has the fitting but nothing mounted in it. That is interesting for a very good reason...it was the 179th production aircraft and never went NEAR a test establishment as far as I can tell. However it WAS one of the last aircraft fitted with a Merlin II and obviously still had the fitting but no instrument/device when crashed in August 1940, we know that P9374 and N3200 (Spitfire I, hand pump U/C but no fitting when crashed in May 1940) had Merlin III and NO fitting. Interestingly, BOTH have certain features (but not the same as each other!!) of the aircraft that are obviously designed to acomodate this bracket (drill holes in stiffeners etc etc...)
So, as the Prototype is the only cockpit photo with "something" fitted there and the early Merlin II's had the bracket , I am thinking now that it is actually some sort of "master cylinder" or similar, if not a pressure instrument with a diaphragm. Maybe an oil flow "sight" glass, or something was included. We KNOW that there were systems early on where visual indicators were duplicated... landing gear down/up, flaps down/up... perhaps this was another case like the flap gauge, of something that could be told already by other indicators (Oil pressure and temp gauge?)
Whatever it was, it was important enough for the attachment points and pipe hole to be included on the drawings of the early Spitfire I instrument panel and to have the bracket attached to all early aircraft (?? looks like it anyway) but NOT important enough to keep fitted for the whole (any??) service life.
So still a mystery and even more potential candidates now...but from my point of view at least the bracket seems like it would have been in K9817, So i get to make one, even if I don't know what it damned well does!