I measured the holes through the vertical part of the seat and got .375 (3/8 diameter) - - but there were no nuts or bolts present. The seat bottom holes measured .312 (5/16), but they were pretty much destroyed. they are different size diameters in my pan.
No biggie though. I'll just make them all 3/8".
Gotta love fixing someone elses crap work. Just another in the long line of doing so!