PRI lo-pro (a couple of them), JP (A2 front-post type), got an Armalite factory block on one of them (the SASS is bad ass). I'm currently in search of the elusive Noveske switchblock for 7.62 for a 12" AR-10 build I'm working on. Those things are hard to find. There's other manufactures', it's all in the type of feature you're looking for (set screw vs rotating knob type), material the block is made of, and of course.......cash that you're willing to part with.