Glad to read that. Enjoy your new faucets!
Just for the record, the mystery O-Ring identified as item 3 in the 525SS diagram is the seal for the "forward sealing" ball valve. Installed, it resides inside the faucet spout, just beyond the valve lever port.
The reason they placed it where it is in the diagram is it has to be inserted through the coupling end of the faucet (with the valve lever removed) and pushed all the way forward to its home.
Don't know why this didn't register with me earlier, but I was
drinking beer at the time