In 1961, the APX-6, although, I think, were still in use, had been eclipsed by the APX-25 IFF system incorporating the KY-25A (or was it 95?) IFF/SIF encoder. The two units look very much alike. By then Mode II (mil only) selective code, was necessary. The coder had a 6 bit selectable address yielding a two letter identifier.....wowee!!