I agree with Lang, and the Clinometer Case in the background is the same as that which equipped both the 25pdr and 18pdr (all marks) from WW1 until the final withdrawal of the the 25pdr in the 1970s. It was carried on the inside face of the shield.
As I understand it, the hand clino was used to cross check the calibration of the inbuilt sight clino. Each time the sight was mounted (it was demounted and carried in a case when travelling), the hand clino was placed on the mark in the top of the breech block, and the two compared and the sight clino adjusted accordingly, to ensure the sight and barrel were in perfect 'cinque'.
Also used by the safety officer to check the angle of the barrel prior to a shoot in peacetime, and I believe a version is still in use for this purpose. The digital age is one thing, but physical checks are still required and carried out to ensure the rounds will impact where intended. A small deviation in angle at the gun will cause a round to impact short or long of the target by many meters. The danger of this is obvious.
Mike C
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