sr_mechanism_model contains the transmissions used in the robot model. We needed specific transmission as we're using our own actuator. We also needed to take care of the joint 0s which combine the distal and middle phalanges.
The transmissions inherit from the pr2_mechanism_model::Transmission class.
They are used to propagate the data from the joints to the hardware interface (and vice versa). For more information, you should have a look at the pr2_mechanism_model wiki page.