flathead 48:
My stock 40 seat is bolted to cage nuts on the underside of the floor. There are two 5/16-24 bolts on each "rail" of the bench seat. Bolts appear to be Grade 5 as they are easy to cut with a file. I am assuming back in 1940 Henry Ford did what was cheapest to do. You figure, seat belts didn't arrive until 1960's. Check out the load strength of Grade 5 5/16 bolts.
https://www.google.com/search?q=5%2F...hrome&ie=UTF-8
It takes thousands of pounds of tension to pull a bolt from its threads. In fact, I have read that Henry Ford used lots of Grade 5 bolts in areas where car folks use Grade 8 bolts so that they would actually stretch rather than snap off. I bet Henry's seat to floor bolts were Grade 5. Plus, it's cheaper.
If your seat belts came from the factory bolted to the frame, some engineer thought it was a good idea at the time. But, times change.