Veggie, the waveforms of the ST heads are often usable, with THD of 15-20%. (Poor for generators.) Some have terrible spikes on the peaks which cause trouble for electronics, some have very bad ratcheting or discontinuities/spikes along the waveform. For running 1/4 hp or more induction motors, none of these will be problematic though some small motors (as in timers) may moan and get hot.
The waveform distortion variations seem to be caused by the stator windings, of which there are many different versions. I had 3 different ST-3 stators on my bench at one time, and no two were the same, even in physical dimensions of the laminated core. This seems to be a situation where China is running head to head with Rajkot in variability and lack of QC. Even recent units from CGG exhibit wild variation including improper (high) harmonic output to the point where an AVR isn't optional. (Although that unit has a decent waveform.)
There is no way to tell what you have short of looking at it with an o'scope and load testing it. Even the cheapest handheld digital ones will work and are a useful tool.