Haven't seen a cover like that, but the hole pattern appears to match what I would call " a crows foot fixture stud " which bolted to the bottom of 8b' and 1900 boxes then could screw fixture stud extenders to support lighting fixtures.
#10136 - 06/02/0209:51 PMRe: What is this used for?
Many years ago I hung a bunch of pendant fixtures that used these. They had a strain relief that attached to the plate (like a crows foot) and a canopy that slid up and snapped on the strain relief and hid the whole thing. Haven't seen these for a while.
#10137 - 06/02/0211:43 PMRe: What is this used for?
Besides lampholders, the more common use of this cover plate, in my experience, was for mounting the old round, porcelain-backed twist switches. The narrow slots accepted the mounting screws (bolt and nut) and the wider "clover leaf" allowed the conductors to enter the holes in the back of the switches.