Herkenning van de M1
The US M1 has been in service since early '40s to early '80s so what distinctive features can tell us wether we have a wartime helmet or  a postwar one? Let's start with some basic rules:
 - The shell -
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. 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. Then the seam moved to the rear until August 1945 when production ceased.
3) the production of M1 shells started over in 1951, so how can we recognize a late war from a postwar? Under the brim there is a number indicating the production lot. If this number is between 0 and 1300 then we have a WW2 era shell. This is true for McCord Radiator manufactured shells, however all Schlueter shells have been produced during WW2.
4) and last, also the color, the texturization and the shape of the shell indicate its age. Wartime shells are a little taller, in a darker shade of green. In postwar helmets sand is used to texturize the surface instead of cork.   
A distinctive characteristic of early war examples, due to the type of steel used, is that they were prone to generate stress cracks on the shell. Also they were fitted with a rim that lost the paint showing the brightness of the stainless steel it was made of.
- The straps & the buckle -

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. However straps OD#3 can be found in rear seam late war examples too. Postwar helmets have attached straps colour OD#7.

Early war examples had a raised bar brass buckle. Mid war helmets were fitted with blackend steel buckle with simplified design. 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.

- Field and postwar repaint-

Most of US M1 in use during the Korean War up to early Vietnam War were produced during WW2 and then repainted, fitted with new straps and new liner. It is not easy to tell if the repainting of a shell dates back to WW2 or later, other clues can help like the stitching on the straps and the liner. See below some examples of repainted helmets, they may be either WW2 field repaint or postwar refurbishment of original WW2 shells.

- The liner -

According to what we said above the US M1 may result in a mixture of components added or replaced in different years. This is true for the liner as well. We can just identify some criteria to distinguish a wartime liner from a postwar one.

First of all the producers: Low pressure liner Saint Clair and Hood Rubber, fibres liner Hawley and General Fibres, high pressure linerInland, International Molded Plastic, Seaman Paper co. and Firestone ceased the production before or at the end of the war. All other high pressure liner: Westinghouse, MSA and Capac were produced after 1950 too.

The straps and the A washers: during the war the straps of the lining system were in tan colour OD#7 whilst postwar liner had green OD#3 straps.
The leather chinstrap: another component that can help to date a liner is the leather chinstrap. All postwar chinstraps are made of blackened steel, are marked DOT on the rivets and usually bear a anchor under the flip tab of the buckle.
Below you see an early fix type chinstrap; a midwar green hardware chinstrap marked "United Carr" and a late war (1944) brass buckle with patent number "United Carr" chinstrap.