I believe Stacy is correct. Those older foredoms (at least the ones I've seen) have the same brushed universal motor as the SR series, and the typical wood router. Now their are different ways to achieve speed control on universal motors, and I'd rather expect foredom uses one of the more advance ones (I know for a fact the HF speed controller is the most cheap and dirty thing possible) but if money is an issue, the harbor freight should work, and should do it without hurting your tool. You may end up loosing some low end torque, but that likely isn't the end of the world.
The other option would be just not using a speed control. My dad has a rather ancient foredom without one, and even my new SRM (bench model with speed control knob) is on full speed 99% of the time. I personally would see a lot more use for variable speed in jewelers work, than what the typical knifemaker uses them for.