I seem to remember British Midland having an agricultural type tractor at LHR in the eighties, which I am pretty sure they used on some of the lighter aircraft (although it's a long time ago and the memory is a bit fuzzy). I'm sure somebody from BM ramp can confirm this. Commercial aircraft bars have torque pins to prevent pulling\pushing damage, and axial pins to protect turning damage. The smaller GA bars I have seen do not have shear pins, but exec jets certainly do.
The bars have an eye secured by a pin on the tug, which obviously allows for some movement, but a good tug driver will push the pin against the front of the eye before applying power to give a smooth start to the push.