Looks like a German trailer that was turned into a .50 cal platform.
4th Tanks, 4th Marines, Co. H&S
44MB Dutch Army Lend Lease Return
44MZ-2 Marine Radio Jeep (under restoration)
43MBT Square Tac. Mark  U.S.M.C. 83371
42 G7117 Chevy 1.5 ton
42 Ben Hur, and Kelvinator G518 Trailers