They appear to be nice quality turn of the century (20th) oak dining chairs that have had their upholstered slip in seats replaced with random boards.
Replace those rough planks with a nice padded slip seat and I think you will find them more comfortable.

We’re not sure the seats were Upholstered. If you look closely, there’s no indication something was removed and there is a groove along the edges. Thanks for your comment. Any one else?

There is no way those lovely chairs with the fancy back would have had those rough seats with the random boards and the nail showing.

Could it be that someone tried to make them "French Country" by putting on those rough seats? Back in the day when it was a popular look

If you look at old chairs with wood seats, they almost always have at least some type of contouring on the seats. Yours are so rustic, including the backs - except for the piece in the middle, they almost look like someone cobbled together a couple of different sets making repairs or partially home made them and put those seats on. It looks like you can see nails where the center of the back was nailed to the cross piece.

Unless you plan on restoring them, I wouldn't feel bad about separating them, but if anyone is interested in buying them, you might have better luck selling all 6 and buying 2 comfortable ones for yourself.

They look like they've been left out in the weather, or lived through a flood?