The purpose of this thing is to keep the lower strings in place on a Precision Bass headstock, becasuse the guitar desing results very straigth string laying. When playing hard, the low strings may jump out from the nut. As I had this problem, I decided to buy the string retainers, originally for a Squire CV70 bass. The tuners of that guitar are simply pressed and glued into position with the small screw on the back side. Also the Squire tuners diameter is bigger than the retainers, so had to enlarge the holes. This cased some scratches (my bad, though!). When I finished assembling the string retainers and tuned again the strings, the tension picked up the unfixed retainers and resulted a disaster look.
Later moved the retainers onto a Fender American Performer. Its tuners have big fixing screws on the headstock top side and these screws helped keeping the retainers in place. And in this case they work perfectly, provided a good slope for the lower strings, so they did not jump out from the nut during playing.
The tuners' mechanic is recommended to consider before purhasing the retainers. If OK, just buy it and enjoy the slope of the E-A strings.
Recommended.