1941 leather case.

[Tony Smith]

The Broad Arrow doesn't necessarily indicate Military, it was used on all sorts of Government property. I have seen it used on Telephone equipment, land and survey equipment, even fair trading/weights and measures kind of stuff. To my eyes, those "lockable rings" appear too delicate for military use.

That said, it could be for some sensitive instrument like a theodolite scope or electrical gauge like a voltmeter or ammeter that wasn't expected to be exposed to the rough and tumble of usual military usage in the field.