Interesting. The older model 4s (like mine) have a 1793 -- the floppy controller board is indentical to that in a model 3. I'd assumed they used that board in the gate array versions as well. Maybe they did (in the early gate array machines), and then changed to the 1773 for the last ones. I don't see any reason why that couldn't have been done -- as you said the 1773 and 1793 are pretty much compatible.