The use of clasps is postwar.2) all shells with the rim that has the seam in front have been produced during WW2, from April 1941 to November 1944, either fixed or swivel bails.
Since the production began, until 1943 the colour of the straps attached to the shell was the olive drab #3, from 1943, mostly on Schlueters, the shells started to be fitted with straps colour olive drab #7. Mid war helmets were fitted with blackend steel buckle with simplified design.
1) all the shells with fixed bails to hold the chinstrap have been produced during WW2, from April 1941 to October 1943, then started the production of swivel bails.
In both cases the straps are stitched to the loops.
However straps OD#3 can be found in rear seam late war examples too. Late war helmets have a blackened brass buckle with simplified design.
Since September 1944 a new release hook, called T1 was adopted and seldomly mounted, though it became a standard only on years '50s production.