Separately inserted air vents are a common characteristic of M1935 combat helmets.  The stamped (sometimes referred to as embossed) air vents on the M1940 and M1942 helmets were created to save the additional work and materials associated with inserting the M1935 style air vent into the helmet shell.  The M1935 style air vent also had a habit of separating from the helmet shell during hard impact thus becoming a projectile itself.  Many stamped air vents vary in their overall shape and look.

Appraisals

Combat Helmet Types

Glossary

Quick ID Charts