[JesseK]

Quick answer just in case you're in immediate need, then others can follow with the real data later--

The two black holes are #10, and iirc they're spaced ~2-1/8" apart, or just slightly more than that I think; I only had a ruler with 1/8" increments when I made a mount this passed weekend and I had to slightly adjust the holes I drilled.

The third, metallic hole seems to be the right size for 1/4" bolts, yet neither 1/4-20 nor 1/4-28 bolts fit...which leads me to believe all of the holes are meant to be metric.