Thats's pretty much what I remembered on my two 428 block's . It appeared to be a cotter pin in a plug. Factory used pound in soft plug, but anybody that had to live with it in the event that those plugs might come lose, would have converted to threaded plugs. I figure it was there to, as you and I said, evacuate the trapped air out of the system radidlyat start up and then flow would slow once
oil arrived. The pin was supposed to move around a little so it wouldn't plug closed with sludge and or varnish. Resulting in a controled leak that you could live with.
The article was more information than I would have found on my own. Thank you
On the old FE blocks 332,352 390, and so on pre 63 center oiler .there was an
oil gallery coming up from the pump across the face of the block and when you converted the cam from the old cam with the spring and button to the newer 63 on up with the thrust plate. The two
oil gallery holes had to be threaded for bolts to hold the thrustplate One of the oil gallery holes that had to be threaded for a bolt had to have a groove groun in the side of the threads and the length had to be shortened to not block the oil flow from oil pump to the gallery. Oh by the way that also lubbed the distributor. I had a friend that haistily converted an older 390 and stuffed it in. The distributor froze. He had the most of the car off the ground by the distributor before it let lose. Same guy lost that special cam bolt so he used a shorter bolt in a Ford 324° cam. The bolt was only into the centering snout that centers the escentric for the maechanicle fuel pump on the camshaft andthat snout broke off in Kansas City. He had to be towed back to DesMoines Ruined a perfectly good 324° cam.