This information was excerpted from an email from historian Bo Lojek


Joe, my 6104 is identical as K2-P. Black label GAP/R MODEL has a silver

window with hand written number 6104. The opposite side read: George A.

Philbrick Researches, Inc. Boston 16, Mass, - 105503 - Made in USA. 



The date code for this 6104 chopper amp is 6174. The chopper is an OAK type.