That is a pressure port on the oil pump. Originally, the line to the oil pressure gauge plugged in there directly. When the partial flow oil filter was added, that special brass fitting was installed. It is a restrictor fitting to limit the flow though the filter. That just means it has a smaller than normal passage in it. It also has two flare fitting outlets. One (the steel line) runs to the oil filter. The other (the copper line) runs to the oil pressure gauge. Copper is not really a good idea for oil lines, when vibrated it tends to work harden and crack. I probably should replace that someday.

Sorry for the lack of pictures here. For some reason I just forgot to take them until I was nearly done. The first thing I did was lift the car and remove then engine pan. It was not obvious where the oil was coming from. So I took loose the two oil lines and removed the brass fitting to inspect. I was really concerned the fitting might be damaged. I don’t know where to get another one. But the fitting looked good. The only potential issues I saw were that the NPT threads on the fitting had no sealer or Teflon tape. I also thought the flare on the copper line looked janky. I pulled the line out, which was not as simple as I would have liked. The line is snaked along the block behind a bunch of stuff. Then a bell housing bolt needs to be removed to release the clip holding the line. Finally for good measure the other end is kind of wedged between the head and the firewall so getting two wrenches on it was a huge PITA. But I got it out. One the bench I closely inspected it and could not see any reason for it not to seal.