The RPM REAR A-ARMS for the ARRMA KRATON, TAILON & OUTCAST (BLACK) (part #81402) is a heavy-duty, engineering-grade nylon rear lower suspension arm pair designed specifically for V1 through V4 ARRMA 6S BLX platforms. 
They replace stock ARRMA (#AR330249) arms, cut unsprung weight by roughly 10%, reduce hinge-pin slop, and add a second outer shock-mounting hole to lower ride height.
#81402 does not directly fit V5 or EXB 6S trucks (which use thicker arms, shorter sway bars, and different shock pins); if you own a V5 or EXB 6S model, you need RPM #81722 instead.