Ball bearing string retainer installed. Normally this would be a flat disk model (vintage) or those flat steel pressed “butterfly” models you see regularly, but both of those can have quite sharp edges.
These on the other hand, have smooth round gliding surface and are therefor less likely to cause string breakage.
It is held in place with the screw and a little “plug” type protrusion on the bottom. I used official documentation as well as googled images to find out it’s location. The main thing is that you want it exactly between the B- and high E-strings obviously, generally they’re mounted near the A-string post. The internet says around this position is fine and I eyeballed it a little bit.
The screw became a bit marred due to screwing, I solved this by filing it a bit with a diamond coated file. You can also screw it in and out a couple times or lube the screw with some candle wax.
One might argue to mount it the other way round. I kind of thought this looked alright.