Its all to do with motor rotation and the way the motor spins. A lot of testing was completed with the CR2. The original pictures of this buggy was with the motor the same side as the associated picture above. The CR2 conversion was available in both RHD and LHD motor. The outcome with testing was to go for the 3-gear box with additional Idler on motor plate.
This allows for the motor to spin the same way as the layshaft and spur gear. If you just reverse the motor this will change the way the motor spins changing the handling of the buggy.
You never know Associated may bring this out LHD and RHD being 3-gear or 4- gear. Only thinking ahead............
I was only curious at the start to see what Associated had done with the gearbox as rumor had it the S2-EVO was going to have the Idler on the motor plate and just wondered if Associated had done this...........