Stock mounted cartridge holders for the Modelo Polylactico in 43r in 5-9 round versions.
Install with the larger flat edge up, as it serves as a ledge for the rim to rest against.
Intended to be installed with #6x1/2" flat headed screws. Larger screws may not sit low enough to allow the shells to fit. The holes are undersized so you can use them as a locator to drill pilot holes into the stock.