It was just what I read on the National Valve Museum Website!
http://www.valve-museum.org/
They have equivilants listed with a picture of valves they have. Another useful source of info is the TDSL on the Duncan Amps site:
http://tdsl.duncanamps.com/
You can download it to your computer, and it is a useful little thing to have-it gives ratings, short data, and links for quite a few types of valves. It isn't exhaustive though!
Sam
*added* After I properly read the whole thread, XTC mentioned the KT61 here. Sorry!