This auxiliary cartridge was made by Frankford Arsenal and is unmarked.  This was used in the end of a rifle grenade launcher to increase range.  This has a steel case and was originally packed in 5 round boxes sealed in an paper envelope.  The label on the envelope read: "5 Cartridges,  Grenade Auxiliary M7, Ammunition Lot FA S-23, Frankford Arsenal,  Loaded Aug 18, 1944".  This interesting little grenade cartridge is the smallest launching cartridge I remember seeing.  Free shipping.