The round helmet goes back to WW1, and was used by British and American troops. It was still in US service at the time of Pearl Harbor, but the switch to the M1 helmet was in progress. In US service the WW1 pattern was the M17, and the later version with a new design of liner was the M17A1 In British service it was partly replaced by a new pattern by the end of WW2. This is what you'll see in photographs of the Normandy landings. The old type is sometimes labelled as a "Brodie", while the 1944 pattern is sometimes called a "turtle", and was still issued at the time of the Falklands War. In British service the paratroop helmet was very different to both, and the basic shell was also used for tank crew and other specialised helmets. So, with a bit of attention to detail, you could have quite a varied collection from one basic model.