Hi Keith71, well done on getting an ST head. Once you have taken delivery, you need to check it out completely. The wiring in the dogbox on top is usually very poor, the little bridge rectifier will probably be rubbish. I replaced the rectifier with a decent unit bolted to a heatsink to keep it cool. Later, I fitted an AVR.
Check out the two bearings, these are often unsealed cheap Chinese sh1t lubricated with yak fat.
Check the slip rings, these are fixed to the shaft with an adhesive and are often not concentric with the shaft. I hade to put mine in a lathe and skim them to get them true.
The brush holders are nothing to write home about but they do work. The brushes are very soft and brittle I recommend you get some spares if you need reliability.
Bob