An endstop holder for the microswitch-type endstops. In my machine, the Y axis stop mounts just above a linear rail which is in turn mounted to a piece of 2020 extrusion. This part mounts to the 2020 extrusion and runs over the top of the rail, where the stop is mounted. I got this printer secondhand, so I'm not sure where the stop comes from (or the design for the mount that broke which this replaces), but I thought I'd share what I made for anyone who needed one of these.
Edit: I found the source. Thanks to Rajaa for the original idea!