Cal 23M dated to 1945, 33mm, matching case back and lug numbers. Will contact Longines next week for an EoA. It came with what I suspect is a period box too.
Edit to add EoA: The original serial number 6'911'xxx identifies a wristwatch in stainless steel bearing the reference 5403 and the order number 22969. It is fitted with a Longines manually wound mechanical movement, caliber 23M and was invoiced on 21 September 1945 to the company Wirth, which was at that time our agent for Switzerland.
I was expecting a ref 5482, but I suppose that was only because I had seen a same size case 'error proof' dial on a ref 5482, and no other reason really.