And two day later.
It's leaking again.
But from a different seal. Seems like just the act of opening up the system to even look at the O-rings on a pump this old causes them to fail on reassembly. The more I mess with this the more I'm starting to think that getting a rebuilt one from autozone for ~$120 with core exchange is not such a bad deal. This is really frustrating. But "In for a penny; in for a pound". I soldier on.
Here's a photo of the hex fitting where my so called mystery O-ring probably rests. I am not opening that up.