Officially called "Case, Magazine 30 round, with shoulder strap".

The Thompson mag bag was a popular pouch to carry spare magazines for the Thompson SMG. It has a built in shoulder strap so it is easy to carry.

The pouch has a flap which is secured by a lift the dot style fastener and it holds the longer 30 round Thompson stick magazines.

This is the type of pouch that was worn by Tom Hanks in the film Saving Private Ryan.

The Thompson Magazine Bag for 30 round stock magazines features:

    Shoulder strap sewn onto the back.
    Adjustable shoulder strap maximum length is 52" (132cm).
    Holds 6 x 30 round stick magazines for the Thompson machine gun.
    Flap is closed by a lift the dot type fastener.
    Made from cotton canvas.
    Colour is OD#3 and is slightly darker than some of the really light coloured tan replicas available.
    "U.S." stamped on the front.