Here's a great page on another site:
Hey Jeeps -I found this on that page:
LATE WAR M-1 HELMET SHELLS
M-1 Helmet (1944 - 1945)
Due to the profound problem of the brightness of the stainless steel rims, in May of 1944 the Army Service Forces requested that the problem be corrected. Until that point, other options to solve the problem had been implemented such as the use of sandblasting and utility tape, but these both were unsatisfactory.
Distinguishing features
In October 1944 the rim material changed from stainless to the same steel that the helmets were made from (Hadfield Maganese). Evidently, for some unknown reason, at the same time, the weld seam of the rim also migrated to the rear.
Note: I personally have found numerous rear seam WWII era shells with stainless steel rims, suggesting the existing stocks of stainless rims must have been used up even after the change was put in place
Proud Daughter of Walter (Monday) Poniedzialek
540th Engineer Combat Regiment, 2833rd Bn, H&S Co, 4th Platoon
There's "No Bridge Too Far"