Well, it has taken nearly two years of backwards and forwards but I think we have solved the mystery.
Richard from Airtech pointed out the other night that he couldn't see evidence of a Kygas Priming Pump on Frame 8 on the Prototype, even though there is clearly a 3 way Primingcock on the frame. That got me thinking and after a long night of searching I stumbled across an old Tractor Fuel priming pump that looked almost exactly like this:
This satisfies the three main conditions that I had on identifying the item vis
1. The need for a hole in the panel for wiring/piping to go through forward.
2. The retaining rings are bolt held, indicating a relatively "permanent" attachment
3. The bracket, towel rack, whatever you want to call it...is physically bolted TO the instrument panel, indicating the need for a certain amount of rigidity.
So it would appear that this is a priming pump, predating the use of the more familiar Kygas and, at least for my purposes, I'm calling this one "solved". The only question I have is whether that pump was fitted to all early Spitfires and removed in favour of the Kygas, or whether the bracket was included on early production Spitfires but the Kygas was introduced in the meantime. On that point, "Spitfire Knuckle" has always mystified me a little as there really doesn't seem to be any significant furniture in the way to hit. But if this pump was in the early production Spits.. it would have conceivably been in the way of the main pump handle.
The most significant clue would probably be had from the early drawings of Frame 8 (lacking the Kygas hole??) but alas, I have only ever found later drawings for the Spitfire I, including the automatic hydraulic landing gear pump. So it won't show that anyhow (nor does it show the Bowden cable routing of the Landing Lamp Control Dipping Lever)
Anyway, if anybody is interested, that is my new theory.
I'm not sure any of the pilots who flew well before the war are still with us to ask, or if they would remember such a minor detail?
regards
Darryl