Cast metal, about 5 inches in dia, (that is standard decking in the background) at first I thought it was a light switch casing from a WW2 hut but maybe it's a little too heavy duty for that. Ordnance related?
I can't see any image or link Ossie...... Must be one if Canberra saw it......
EDIT: Very strange; had this problem two days ago. Now I see two micro squares but no image when clicked.
EDIT 2 - OK, second time around I can now see the pix. No, that's not munitions/ordnance related in my opinion. Suggest you scrape some of the corrosion off the top of the dome and look for markings. Wire brush should do it.
If it was brass I would have said exterior light switch. The ones I have used had a knurled brass knob and the alignment mark was to signify either on or off, can't recall which as it was 40 years ago!
No Amount Of Evidence Will Ever Persuade An Idiot (probably not Mark Twain)
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