This is a bracket to hold the hall effect sensor from this cheap digital tach on my Denford ORAC lathe. I use it along with a couple tiny magnets shoved inside one of the mounting nut slots on my ORAC Spindle Timing Pulley. Alton Brown would be proud of my multitasking pulley.

I don't know what the pre-existing holes were supposed to be used for or even if they were added by some previous owner. Either way, "it works on my machine". =)