There were some educated guesses involved. Parts books showed Buick used 5/16-24 bolts-not 5/16-20 or metric.
The Google search showed there is such a thing as 5/16-20 hardware, but it is rare. That one message thread introduced the idea of the metric bolt being so similar to a 5/16-20. The dimensions check very closely. A 5/16 bolt shank dia is 0.3125”. An 8 mm bolt shank is 0.3150”. A metric thread pitch of 1.25 mm per thread works out to 20.32 threads per inch. It would be hard to tell the two apart unless maybe there are some metric markings on the head.
Whatever those two remaining bolts are, the nuts in the floor pan now are threaded to fit the metric threads. I think what happened is someone in the last 30-40 years removed/reinstalled the seat and decided to replace the bolts in the process. They grabbed whatever size looked right and wrenched ‘em on in. This re-threaded the nuts for metric threads. 5/16-20 bolts are too special and hard to obtain to be considered as the replacements back then. It might be best to just use the new metric bolts if they thread in easily rather than re-cut the threads again back to 5/16-24.