This is the V2 Team Associated B5M/T5M 4 Gear Gear Box, which comes standard on the T5M, but not with the B5M. This gear box uses 5x10x4mm bearings for the top shaft, whereas the B5M gearbox uses 6x12x4mm bearings. Part# 91552
This gearbox helps keep idler and topshaft bearings more uniform, will all using the 5x10x4mm bearing. 5x10x4mm bearings are also less expensive and slightly lighter weight than the 6x12x4mm bearing.
Note: This gearbox does require the V2 topshaft (ASC91606) to be compatible with the 5x10x4mm bearings. But, as an added benefit, the V2 topshaft is compatible with both this V2 4-gear gearbox, as well as the 3-gear gearbox.