Ok, thanks LR.
That Aero Antique website has given me a good idea behind the theory of its use.
The million dollar question for me is if it's possible to trace the history of this specific instrument back any further ?
Iim under the impression that a good many would have been made and fitted. Perhaps in the intervening years studies and notes have been made that might give me a clue of it's past ?
Cheers Andy