The correct box for the HE round was a C274, the C310 being longer for the APCBC.
I have some sides from a C274 which contained practice flat shot and one small section of base which is T&G and stamped C274 I WI&S LTD 1943. As Rob says, likely boxes were made from whatever met the criterion.
I'd be very interested in a box (or two) if you have a spare!
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Adrian Barrell
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