Gents; The "N" series AB heater is just the heater portion only. If you step up you may also use the "W" series heater which has the heater and aforementioned solder spindle included. To use the "W" series remove the existing solder spindle by removing the two small screws under the heater on the overload block. The spindle is spring loaded so will jump out at you when you loosen the second screw. Relax, this is normal! Throw the old solder pot and two screws away and insert the new heater with the two original large screws. Now push the reset button/bar and the unit will be good to go.
Both series of heaters are wound with the same size nichrome wire so the numbers on the heater remain the same. Check the motor name plate, the consult the heater chart and get the heater that as close as possible matches the amp draw of the motor. Do not oversize. Heaters are already sized to allow a twenty five percent overload without tripping. Remember this is a slow operation. The motor must remain in an overload condition for a period of time before tripping. This allows time for the heat to build up in the motor and temporary overload conditions to cure themselves. The fuses/breaker ahead of the starter are there for the instantaneous disruption of power should a serious overload or short circuit or ground take place. A oversized heater will allow the motor to cook and basically provides no protection at all. Insulation break down caused by excessive heat is what this is all about. The reason motors burn out.
By the way, the starter you have is indeed an oldie but a goodie. Should anything happen to it you may have trouble getting repair parts. I would plan on scrapping it when the time comes and replacing with something produced in the 20th or 21st century. Just a thought.
HTH
Just wanted to personally say Thanks, for the heads up on the two heater style overload system! Never came across one of these, before! I do see where it would work perfect, in the normal 3 phase delta setup, but like you said when one leg grounds, to the case, and especially the leg with no heater! Now that's another story! IE=BAD
Take Care Bob in Oregon