The Rheostat does not have to be made by Mallory does it?
You know wattage, resestance and configuration it seems you could find one with same values.
Some of the old Hobarts had a long extension cord attached to the fine adjustment and it just plugged in to the front of the panel, that way you could take it with you like a remote control.
We had a few when I was in the Army (Brown Boot) that got destroyed and we could not get original parts either so we Bast@^%#ed them and they worked fine.