For those of you who remember “Fawlty Towers” (with John Cleese), it is what the Spanish waiter, Manuel, would have said if he, too, had come across this button…“K??” 
(“Qué?”)
I must confess… I feel guilty. Guilty for spreading fake-information.
The mystery of the “K” Button is still solved - this time with real contact to Steinberg officials.
THe “K” is a tribute to Charly Steinberg. How comes it is “K” then? Well - Charles is “Karl” in German. Karl Steinberg. The tribute Button was introduced to give honor to this great person. It is “K” for “Karl Steinberg”. It will be “C” from the next Fix on.
Cheers, Ernst
Okay, it was a joke then.
On a serious note: icons would be better UI design than single letters. It is too arbitrary.
Kaput