Neither of those number Don gave you are what you are after. That is not a screw, it's an eccentric used to adjust the depth of engagement of the teeth between the chronograph drive wheel, and the coupling clutch wheel.
The part is 320-1702 - Eccentric for pivoting of coupling clutch.
Thats why I said to email to be sure. Number is not listed on the parts page unless I somehow missed it
This part was listed for both 321 and 320 - COUPLING CLUTCH PIVOT ECCENTR 1702/OME 27
Emailing and asking for a screw isn't going to get you a better answer. It's the responsibility of the person ordering to know what they are ordering, not the vendor's to figure it out for you (they have better things to do). That's why many will not deal with amateurs, as they don't know the correct names or part numbers most of the time, and suck up a huge amount of time for just the odd part.
Not sure what parts page you are referring to, but it's clearly listed on the Omega parts list from 1958 that is freely available all over the internet - that's where I found it.
Emailing and asking for a screw isn't going to get you a better answer. It's the responsibility of the person ordering to know what they are ordering, not the vendor's to figure it out for you (they have better things to do). That's why many will not deal with amateurs, as they don't know the correct names or part numbers most of the time, and suck up a huge amount of time for just the odd part.
As an amateur, this hasn’t been my experience. I find the folks at the material houses very helpful and willing to sort out these type of questions if you bring them even a rudimentary amount of information about what you’re after.