The 1772 and 1770 were designed specifically for use with the 5-1/4" units, though that's not necessarily what their use today may be. I've never seen the 1773 in the flesh, though I've got a data sheet somewhere (though I've managed to hide it somewhere). The three chips are all similar, though not identical, in that they have the same data/clock separator circuitr and write precomp circuit. However, the 1773 lacks some signals that are on the 1770/72, thereby rendering it less "dedicated" to 5-1/4" drives. The datasheet did, IIRC, specifically indicate that they're all similar in commands, etc, though the timing parameters differ somewhat in that the 1770 is slower than the otherwise identical 1772 in terms of its step rate settings. If I could lay hands on the 1770/2/3 datasheet, I could tell you which pins differ and in what sense, but ...

If somebody's got a 1773 and wants a 1770, I'll happily swap 'em, just so I have one in house.