The helmet has been coated with a rust-encourager, which has worked well. There are discussions of how best to remove rust on the USMF, and some have had marvelous results. If it's a front-seam, it's WW2 vintage. If not, it can be hard to tell - though the Navy used the same helmets for decades.
The liner has the OD3 khaki webbing indicative of WW2 but has a Vietnam era sweatband (darker OD7 green). The paint and decals indicate that the liner lived a long life through the 1960s at least.