Desperately seeking a size 36 N424-21 Pilot jet for ET4 ISO, I stumbled upon the answer to a mystery described Lo these many moons ago. What is the significance of the "S" behind the Size 35 Pilot Jet that came stock on the S50?
Power Barn in Wisconsin has a website that shows a side-by-side comparison of the Keihin jets. (For Honda XR650's "Secondary" carb, in this case... Yes the 600-650 singles had Two (2) carburetors. Not fun to tune.)
So the "S"-marked jets have much smaller emulsion holes in the stem of the nozzle. Evidently this was to improve fuel atomization when low-intensity of vacuum pulses were present. Mystery Solved! Not sure why Punkin's 4-valve engine had such low-intensity pulses, but that's the answer.