BSS KZ stub axle is for 2015 and newer OTK shifter chassis with the BSS brake system.
HST BS5 stub axle is for 2014 and older OTK shifter chassis with the BS5 brake system.
HST SA2 stub axle is for all OTK non-shifter chassis with 22mm front stub axles.
BST stub axle is for all Rookie, Mini and Nordix OTK chassis.