This is where I got confused to start with.
As mentioned I have searched various websites and publications (e.g. Mustang Recognition Guide 1965-1973 - by Dobbs, Farr, Healsey, Kopec).
This clearly shows pictures of the radio with chrome push buttons for 68'.
Various car sales web sites showing 68' cars for sale show chrome or black push buttons for the radios.
Could it be that 68 had chrome and switched to black during production cycle?
regards