Basically it's a lower specification KT76 A with lower power output and it is unable to code mode C above 15000ft.
I would guess that it was built as a low cost unit to use parts that work but don't meet the spec for being fitted to a KT76A, King also did this with the KN62 DME, when unit failed meet the KN62 power out put they badged it as a KN64.
If you got lucky your KN64 had only missed the spec by a few watts and you had very good DME in terms of output for a budget price.