I changed mine to socket-head metrics - M8 I think, that's the equivalent size to 5/16. The socket head makes it much easier to undo/tighten them, and in a situation where both bolt and nut are used switching to metric makes life a lot easier when it comes to carrying tools. As for originality, I'm afraid I missed that particular boat with my car about 20 years ago :-)
One other tip if one is contemplating removing one's seats on a regular basis. If you take a 14mm Forstner bit to the underside of your wooden battens, and use capscrews rather than bolts, then you can bolt the battens to the underside of the seats and have all 4 capscrews in position ready to drop straight through the holes in the floor. Makes it a lot easier to fit and remove the seats as you then can tighten up from underneath without having to hold the bolt in position.
