Don't remember ever using Korean tubes, but those International Servicemaster tubes were a REAL crapshoot, especially toward the end of the tube era.
As the major US manufacturers went out of business, they started sourcing tubes wherever they could find them, and rebranding them. Sometimes, they did some really sleazy stuff, like branding common tubes with a different type number for a vaguely similar type, which could be sold for more money. Examples here:
http://www.vacuumtubesinc.com/Vacuum.../Fake6JK6.aspx
http://www.vacuumtubesinc.com/Vacuum...arked5965.aspx
http://www.vacuumtubesinc.com/Vacuum...rked12SJ7.aspx
http://www.vacuumtubesinc.com/Vacuum...sian6L6GC.aspx
http://www.vacuumtubesinc.com/Vacuum...rked6Y6GA.aspx