- THE B5 SYSTEMS MODULAR PLATFORM STOCK FEATURES AN INTEGRAL LEFT SIDE FOLDING STEEL HINGE, ADJUSTABLE LENGTH OF PULL, SPRING ASSIST CHEEKPIECE, AMBIDEXTROUS FI XED AND LIMITED ROTATION QUICK DISCONNECT SLING MOUNTS, AND NONSLIP BUTTPAD. ATTACHES VIA 1913 PICATINNY MOUNT. CONSTRUCTED OF DURABLE COMPOSITE