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The YHM Nielsen Booster Assembly replaces the direct thread adapter on a YHM Sound suppressor or any other suppressor on the market which utilizes the 1-3/8-24. They are designed to be used with handguns and barrels which require a Nielsen Piston spring. - Replaces your Direct Thread Adapter - 17-4 Stainless Steel - Matte Black Finish - 1-3/8�-24 Mount Thread Pitch