Ok, I've found a couple of references. That connector is compatible with newer SENSATEMP handsets but ignores the tip temperature feedback. So you can use an SX-90 and manually control the temperature. It is interesting to see that the unit was apparently serviced several times, given the tamper paint on the screws. That's a good sign. Someone took care of it. The earlier handsets were not labeled other than "Sodr-x-tractor". The various revisions 55,60,70 did not change much from one to the next. I would still say, given the color of the plastic and label, that yours is a 70. Also, the relative lack of "burn" on the heater makes me doubt that it is as old as a 55.