Reiner turns on a cylinder record player. A cylinder record player plays a wax cylinder. On a cylinder record player, all the parts needed for reading the record are mounted on an assembly that moves with the record, including the horn. Not only is there no wax cylinder on the record player, the sound is coming out of a horn to the side of it instead of where it should be coming out of: the horn mounted on the assembly.
When the German Storm Troopers, complete with metal, bulletproof vests, ambush the allied unit, one of the German's is carrying what appears to be an MG34 or 42 light machine gun. Greatly feared by the allies, it wasn't created until the 1930's, while this movie is set in 1918.