It is a Mattel Shootin Shell Sealed Package. These are the very special yellow tips and 3 brass.
Push bullet nose prongs into metal casing, as shown and the two parts will automatically lock together into one cartridge
Then if you want, press a Greenie Stick-M-Cap on bottom of cartridge
Don’t have Greenies??? Check the web and yo will find 120’s.
If you are a lost in the 50’s Cowboy who loved to play with the Mattel Shootin Shell Cap Guns, then you must have this package for your shooting and collecting enjoyment