Thanks, all- something's definitely up here (aka wrong threading, etc).
I've never had to do chasing or tapping before, so all of that's new for me.
As for the questions re: dirt in the threads etc, it has a mild dusting of primer on them, but I didn't see any real "dirt". Also, as they are interior holes, I could see the possibility of there being *some* dirt or corrosion, but I wasn't expecting this!
Also, re: the question about seeing the difference in threading between coarse and fine, normally, yes, I can distinguish between the two- but, these holes are small, and the threading starts a bit further in, so it's a bit hard to tell for sure.
And re: my "gear head" card, I should have earned that already, as I put an entire 1966 Chevelle back together from the ground up, myself. And during that project, I did not need to tap or chase a single bolt hole! This is just da** wacky!!
Lastly, I can see where a prior owner apparently tried to cut out this raised support for the seat mounts. I can't really understand why they did not follow through. It's only on the edges, so for all I know, maybe they were like that from the factory that way? I'll have to get a pic and post it.
Many many thanks for all of the replies, and I'll keep at it.
