Original Zinc/Aluminum training clips or the M1 Garand issued by the Greek Military. It pushes down and locks in place just like a normal clip allowing the trainee to practice without having to use live ammo. It's symmetrical so it doesn't matter which side is inserted facing 'up.'
It also serves as a single shot device. The top is tray-shaped to center the round with the chamber for each shot. Cartridges must be manually loaded by retracting the bolt as the bolt is not held open after each shot. To remove the single shot device, you must actuate the clip-relase on the Garand