SWAG:
Looks like a locating hole for a dowel pin- check the engine to bellhousing mating area to see if it may have been forced together without being aligned, breaking that piece off the bellhousing...
Thanks.
Doesn't seem to affect the clutch linkage, thankfully, it feels great now that I completely rebuilt it.
I better check the bellhousing to motor screws, though, I've found a bunch of loose fasteners so far in my journey.
Whoever worked on the car in past was not very detail oriented.