Factory Swiss SIG 551 Gas Piston rod and spring in like new/new condition.. These rods have a captured braided wire recoil spring. These units may have some carbon marks on the piston from test fire Modern version with 2 holes on the piston head. This piston functions in both the SIG SAN 551 as wells as SIG USA 556. The piston locks into the bolt carrier by way of the charging handle and operates the gas piston system.