It looks like it belongs to a 1971 onwards Mustang judging by the knobs. It will, however, fit a '69 onwards (just change to the correct knobs).
You can tell exactly by the markings on the side. If look at the radio below, the D3xxx part number refers to a 1973 build date.
C9xxx = 1969
D0xxx = 1970
D1xxx = 1971
D2xxx = 1972 and so on.
Most early Ford parts are numbered this way.