Nautilus FRP / PAC-FAB / Pentair Pool Filter O-Ring question

Does anyone have a Nautilus FRP / PAC-FAB / Pentair Pool Filter? I bought a house with a pool. I am getting ready to clean out the DE filter tank again, and I would like to replace the O-ring or gasket that forms the seal between the top and bottom halves of my fiberglass tank. My model number is FNS-36.
Here's my question. I am having a hard time determining which part to buy. I have an exploded drawing showing the parts. It looks like part number 19-5008 is the "Seal -Tank". When I google this, this seems similar to what I need. The parts list also shows a "Back Up Ring," part number 19-5073 which looks like it might go underneath the first O-ring that I mentioned. I do not remeber seeing this last time I took it apart. I did notice that the tank ring seemed to be larger than required and it took several re-seals to keep it from leaking.
However, also on my parts diagram is a note, handwritten by the previous owner, that says 17-4704 Tank O-Ring. When I google this part number, it too seems to be the tank ring. I don't know which one I need. Does anyone else have this model? Thanks, Roger
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