Bumping this thread to add a bit of info that I couldn't find via web search. I recently experienced low gear slipping due to the retrofriction screw not being tight enough, of course I didn't have a big screwdriver with me so I walked a few steep hills and managed the rest of the ride. When I got home I tightened the screw and it held low gear fine, just a bit more resistance when pushing the lever forward. Then I looked into replacing the o/g screws with button head 3mm allen screws. I have braze on shifters, they take 20mm long M5 screws. (The length is what I had trouble figuring out online). I found some at the hardware store. Also, unbranded (mavic?) retrofriction shifters have slightly different pull vs simplex branded retrofriction shifters, I had to adjust the limit screws when I switched from one to the other.