It's not from World War II.
FA stands for Frankford Arsenal, a small arms manufacturer located in Philadelphia.
60 stands for 1960
9 stands for September.
Depending on the length, it could either be the military equivalent of the .308 Winchester (case would be approx. 51mm long) or .30-06 (case would be approximately 63mm long).
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.