so I googled B5M light and up pops ae's website link to the b5m light in part of the description is this
RC10B5M Factory Lite
Team Associated's RC10B5 and RC10B5M have been met with wild success by racers around the globe at both the club level and at major racing events. Always looking for a challenge,
Team Associated's Area 51 engineers took it upon themselves to make the best RC10 to date even better! A version of the B5M that would optimize the platform to excel in classes requiring spec motors and for certain track conditions that allow the racer to take full advantage of the performance offered from reduced weight.
http://www.teamassociated.com/cars_a.../Factory_Lite/