Picatinny interface for the Swiss SIG 55x rifles. This adapter allows mounting of picatinny-compatible stocks on the SIG 55x rifles. It comes with the necessary hardware to mount it (pin).
 
Specs
Picatinny interface for the stock adapter
anodized aluminum
fits US-made 551A1
Fits Swiss-made 550, 551, 552, 553
made by SIG SAUER Switzerland