A number of people I have talked to have seen these cartridges before but didn’t recognize them for what they are. They are the M7 Auxiliary Grenade Cartridges. They were used to extend the range of rifle grenades launched from M1903 Rifles, M1 Rifles and M1 Carbines, although there was a warning that they were only for emergency use with the M1 Carbines. They were not to be shoulder fired with any rifle.
One was placed in the open end of the grenade launcher with the red disc pointing in the direction of the rear sight of the rifle before the grenade was placed on the launcher. When fired, the hot gases from the normal grenade cartridge in the chamber of the rifle would ignite the charge of the auxiliary cartridge and provide additional propelling power to the grenade.
They came packed in a small, unmarked 5-round box which was in a moisture and vapor proof sealed envelope that was marked for the contents. The envelopes were included in two of the three different types of grenade cartridge assortments that came in small “spam cans”.