[Arran]

Terry;
  A true "Variac" is a variable transformer manufactured by the General Radio Company, the rest are just variable transformers. It's sort of like how "Formica" in the U.S, and "Arborite" in Canada became the vernacular for any type of plastic laminate material, or how "Masonite" became the vernacular term for pressed hardboard panels. I have three or four variable auto transformers and I think one is a genuine General Radio "Variac" the rest were made by competitors, one by Ohmite, another is Japanese. Having a 20 amp one is nice, you can have lots of fun experimenting with one of those without having to worry about burning it up, I think my go to one is only 6 amps.
Regards
Arran