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