Sorry if I state the obvious, but the "AM" will be Air Ministry. So British whatever it is.
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is this the only photo , does it turn , unscrew, open in any way , as Ade said it is definately British , i suspect it might be a guage of some description but wait for a few more opinions one of the RRPG guys should know
Looks very much like the sighting optic from an Aldis signalling lamp. If you do a web search you'll see there are slightly varying types of these that were used.
'I do not think we can hope for any better thing now.
We shall stick it out to the end, but we are getting weaker of course, and the end cannot be far.
It seems a pity, but I do not think I can write more. R. SCOTT.
Last Entry - For God's sake look after our people.'
In memory of Capt. Robert Falcon Scott, Edward Wilson, Henry Bowers, Lawrence Oates and Edgar Evans. South Pole Expedition, 30th March 1912.
Excellent suggestion, Ned! Looks very much like a variation of just that after perusing pics of several models of the lamp.
A big thank you to Big Ned.