There are a few things that can help you to tell them apart. For instence, a WWII helmet will be alot darker in color, and have khaki straps. The liner will have a small hole in the front, and also have khaki webbing inside. The webbing in the liner will have a hole in the middle.
A Vietnam era helmet is lighter in color, and has green straps. Its liner will have no hole at the front, and its webbing will be green. The webbing in these liners will have no hole in the middle. On or about 1967, the US produced another 1 million M1 steel helmets and were produced from scrapped cars. These M1's were to augment the diminishing WWII supply, and sent to Vietnam. If the M1 steel helmet has a lowered crown (foreward top portion), and the usual swivel bales, and the usual rear seam, and has an ORIGINAL FACTORY redish/browish primer with an ORIGINAL FACTORY "Olive Green" exterior paint job, non-corked (smooth), then it's one of those 1967 (and beyond) produced Vietnam War era M1 steel helmet shells.
I hope this helps,