If the barrel has an original twist you probably want to go with a left hand twist muzzle device to avoid unscrewing. Original 9/16"×24 LH threads didn't have keys to lock the device while the 9/16"×24 RH models (such as L1/L2/C1/C2 Commonwealth variants) had key ways cut in the barrel for a pinned key in the flash hider to lock the device in place. That choice also necessitates timing washers to line it up.