Try doing a search in the tube asylum over at audioasylum.com. I believe you will find the answer there.
Svetlana KT88 "C" logo vs. "S" logo brands?
I know that Svetlana has undergone some distribution changes here in both Canada and the US. Different suppliers are marketing their tubes as original "S" logo and "C" logo, blue box vs red box etc. and claim superiority over one another. Some even claim that Svet brand "C" are merely rebranded Sovteks. What's the story?