finding when an instrument was made

[toobagrowl]

According to the H. N. White website, your sousaphone was made around 1939 - 1940:
http://www.hnwhite.com/Serial%20Numbers.htm
Another link to old King serial #s: http://www.musictrader.com/king.html

I myself have an old H.N. White King Eb souzy. They are fine old instruments