Yankee Hill Machine Co sRx Low Profile Adapter, Compatible with sRx Muzzle Devices, Attaches to Suppressors with 1-3/8x24 Thread Pitch, Black Oxide Finish YHM-4400 YHM's line of sRx products were designed with aesthetics and ease-of-use as the top priorities" and that is no different for the YHM sRx Q.D. Adapter (YHM-4400). This Low Profile" Q.D. Adapter/Mounting Interface attaches to the industry standard (HUB) 1-3/8"x24 threads. It features an internal taper seal-in front of the threads-that locks your suppressor on and forms a tight seal to contain gasses while in-use. This means no gasses or debris can build up on the threads.