ProMag Industries ProMag, AR Block, Reversible, Fits AR-15, Black PM123 This vise block allows the user a third-hand to work on and install parts on AR-15(R) / M-16 lower receivers. The vise block is clamped into the users vise" and the block is then inserted into the lower receivers magazine well" and is held in place by the magazine catch. Constructed of a black injection molded polymer.