( 30mm case for comparison )
US-made 57mm anti-tank gun, AP-T round. The brass case (dated 1942) has a push-in primer marked 'M1B1A2 AOP 18 1943' and measures 57x442R.
Case headstamps read '57MM M23A2 LOT 34164 FA 1942'.
The base of the case still retains readable evidence of the original stenciled markings.