Since it has no primer or flashhole, and a hole drilled in the side of the case, and is in a machine gun link, I figure it is a training or test dummy cartridge to set up a MG with.
F A = Frankford Arsenal
4 = made in 1944
To save the cost of a new headstamp date "bunter" reading 44, they just ground the 3 off the 1943 stamp. Might have been 1954, etc, maybe a cartridge collector will come along with more.