I have read that the rear seam was produced late 1943 but did not make to the troops until middle of 1944 - Also I think I saw that a WW2 rear seam should be made by Schlueter.
The rear seam helmet had the rim metal changed to use the same metal as the helmet itself (Hadfield Manganese steel ). The QMC did this becasue the original rim was made from stainless steel and this metal did not hold the paint well and the paint would wear off and the shinny metal would show.