So, I did a Yahoo! search for King trombone serial numbers. And the 4th entry that popped up was the H.N. White trombones homepage. Not that hard...
Anyway: Your serial number pops up as somewhere in the 1942-1945 range.
http://www.hnwhite.com/Trombone%20Page.htm<Added>..sorry, didn't finish...
Other than that not much popped up. Looks like an incredible horn though, good luck finding out more about it.