B5 Systems Modular Picatinny Stock, Wolf Gray, Fits Picatinny MPS-1557 The B5 Systems Modular Platform Stock features an integral left-side folding steel hinge, adjustable length of pull, spring-assist cheekpiece, ambidextrous fixed and limited rotation quick disconnect sling mounts, and a nonslip buttpad. Attaches via 1913 Picatinny mount. Constructed of durable composite materials and steel hardware.