Thank you for sharing this. I never saw a windshield adapter and never knew it existed!!!!!The “SD” means special defense. These were used with surpressors and the windshield adapters that allow one to accurately shoot through glass. HK used a 13.5 by 1 LH thread pattern because allows the barrel wall to be thicker than a 1/2 by 28. This ensured that the surprssor is properly supported and is less likely to break off when dropped like a standard 1/2 by 28. Her is a picture of the windshield adapter.
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