The first line of T-verbs (the first year) only had a serial FX loop, which only had one mix knob. The ones after that with the Parallel FX loop had two knobs....one for send, one for return. The earlier ones also had the sunk in "shock" absorbers that mounted the chassis to the head cabinet, where the later ones had that metal strip with 2 screws on each strip. Other than a few cosmetic differences, that's about the only real difference in the beast.
