I just took a quick look at a '71 Plymouth FSM.
A couple questions I have now is
1. how the hot idle compensator works. The text says this opens an air bleed, but does say how.
2. Is there a second air bleed for the transfer slot or not. Description says there is one but I'm not seeing it or a third circuit. Of course the text is covering 4 different 4160 carbs...


As far as the idle mix screw goes, take one out and see if it has a needle valve tip or looks like the ones Mike found.
My guess is that the flat tip worked to increase the effect of restriction within the 270 degrees it was originally limited to.
I'd take a note of how the flat runs, and also be very gentle when closing it to the bottom. It probably doesn't shut off flow completely like a needle tip would. Not that you need to shut off the flow, but when experimenting its common to run the screws in until engine slows.