The spacing on the posts is different for one, and the shape on the sedan headrests puts the mounting posts much farther rearward than the surface of the headrest as the posts go in behind the sedan seatback into the parcel shelf with special mounting brackets. The wagon rear headrests posts are in line with the headrest and drop straight into the holes int he top of the wagon seatback.
This is what the wagon rear ones look like.
Note the mounting post spacing on the wagon rear headrests is the same as the later front seat headrests, but I have not yet been game to push a front headrest fully into the wagon rear seatback on my car as I’m afraid it will snap in and not come out again...I’m going to try and modify the sedan front headrest to fit the wagon by cutting out the notch so it can’t accidentally get stuck in. So that could be an option for you as the wagon rear headrests are somewhat hard to find.