Standard B'port vari speed head made by the 1000's. Can you run it before purchase? If so run it up and down the entire speed range and listen for noises. It should sound smooth without any "thrumming" or other such sounds. There are parts that wear out in these heads and all these parts are readily available, there are also great videos on youtube on how to do all the repair work.
These videos are very good, and the company can supply all the parts you might need.
That said, the head rebuild is NOT a fun task, with parts of it requiring a second person. I just did one last summer.
Good luck with it, hopefully the head is fine!