The SRS-A2 / Covert barrel extension works with all three SRS bolt heads (Short Action .308, Long Action .300 Win Mag, and .338 Lapua Magnum.) The current version of the barrel extension will fit with SRS-A2, SRS-A1, and Gen 2 bolt heads. The SRS Gen 1 has it's own barrel extensions and the Left Hand SRS-A1 has it's own barrel extension as well. Both the Left Hand and Gen 1 extensions can be special ordered.