Bob,
It certainly is a clinometer which just means "angle measurer" but this particular variation is an Abney Level.
All artillery and mortars plus indirect machine guns have clinometers which are a much simpler version of the Abney Level. Before modern digital instruments they usually consisted of a simple brass arc with a level. You set the angle on the instrument then elevated the gun or mortar until the bubble was in the middle.
I am sure the artillery guys can put up a photo of a 25 pounder clino which is very similar to almost every weapon up to the digital era.
Lang
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