SAN SWISS ARMS side rail for the SIG 55x rifles. The rail will be fitted at the side of the handguard and allows for mounting of hardware. See 3rd picture as example on how it is mounted.
 
Specs:
Aluminum, black
compatible with SIG 55x original handguard
Original SAN SWISS ARMS item, no copy.
Rail comes with 2 screws and 2 washers.