I'm certain someone with more experience will give you a definitive answer. From my own casual observations on Bridgeports and other early cnc mills with speed control, it was often achieved by simple replacement of the handwheel with an air or electric motor to achieve speed changes.
A Boss 6 should be of this era/generation. You should be able to locate a vari-speed manual adjuster, if no where else on eBay, that will bolt in place.
Good luck, although I'm certain that you will get just the answer you are looking for from this board.