Over a month later and I have finally figured it out. I'll post my travails in case it might help some lost soul avoid the headache that I endured...
First thing I tried was using a pool rated o-ring lubricant (from pool supply store). That did not work.
Second, I got some instant gasket goop in a tube that hardens into a flexible (not completely rigid) solid. I ran a bead around the bottom and top of the o-ring hoping it might seal any gap. Had to wait a day to let it set. It set well, but did not stop the leak when I reassembled and ran the pump. I ended up cleaning all the goop from the o-ring and shell.
Third, I found https://www.theoringstore.com/store/ and ordered a custom sized o-ring with the same ID as the spec part, but with a slightly larger cross section. It took a few days for the o-ring to arrive. I tried it when I got it and... still leaked.
I ordered a second o-ring from those peeps with the same ID and a bit larger CS. This one also took several days to arrive. When I got it, I almost could not lock the clamp that holds the shells together. This O-Ring for sure would not let water escape between the shell and ring. It would be nigh impossible. Yet, when I assembled everything and ran the pump, it leaked - again from the same spot and with the same flow rate.
I took it apart again and cleaned all the surfaces of the joint. I was pretty discouraged at this point and just really kind of stared at the bottom shell for a while before I noticed that the lip of the bottom shell had a very small - almost too small to see - gap near where the water keeps escaping. The gap was on the outer and bottom side of the lip - *not* on the surface where the O-Ring sits. I looked a bit more and it became apparent that the bottom shell was cast/fabricated as one piece that later had a rectangle cut out of it and replaced with a plate that had the inlet and outlet nozzle connections. There is a line all along the outer shell for the rectangle as if the rectangle had been welded in place (but it's fiberglass, so I know it wasn't "welded"). Anyway I got the bright idea to inspect the inside of the shell for the rectangle lines and inside of the shell was totally smooth - no "weld" line at all. And then I saw it. Just above the inlet nozzle was what looked like 6 inch long horizontal scratch on the inner shell lining. It looked like someone had run a razor blade on the surface. The scratch was neigh microscopic.
So, I applied that instant gasket goop on the inner wall to seal the scratch. i had to use a small trowel to ensure the area was covered properly. I also dabbed a little bit of the goop on the outer shell where I saw the small gap.
I let that cure for a day and then assembled the filter (sans internal assembly / fins) with the original O-ring. Ran the pump and no leak.
The instant gasket goop I used was like flex seal. Good call.