Tony,
To borrow a well worn saying, the only stupid question is the one not asked.
I'm not aware of any pop rivets in the body. There are rivets on the body and frame but not pop rivets, e.g. body: toe board gusset frame attachments, brace between the instrument panel and fire wall, latch on tool box lid. Frame: front bumper, rear crossmember, battery tray, etc.
Not seeing what you're looking at, I'd guess that a PO used pop rivets to cover up an area needing repair with a piece of sheet metal or maybe two pieces of the tub became detached and using the pop rivet was an easy way to re-attach without welding. Sounds like he may have even constructed parts of the tub and attached them the same way.
There's not much difference between the 3A and 3B tub and fenders. Searching through the sections covering Projects here or over at the now defunct CJ3B Tapatalk forum will have images that may cover what you need to see. Also there are many images on the 3B Page as well. The 3A Page also has a nice rebuild project that's pretty well documented.
Or you may get some fresh images here if you could either post what you have or describe what you want to look at and you may wind up with more images than you need.
