Our 4 place seats have holes in the pans to access the two slotted bolts that hold them to the spar brackets.  The earlier models have much smaller holes than the later models but this can be easily fixed.  An example is shown here with the seat holes large and the plastic small.  One way is to cut them out with a blade or hole saw but if you can get hold of a 3 1/8″ knock-out punch it makes the job much easier.  The pictured seat has the large holes already in the metal pan but the covers have been replaced with the smaller ones for some reason.  If you find this, just center the tool through the small hole, to align with the larger one, and tighten it down to punch out the plastic only.  If you have small holes in the metal pan, you can punch it out both parts together or separately.  Flanging the holes in the metal is not necessary but  you can if you wish.