I have complete 67 diagrams that I can lay on the table and study. Don't have same for 68 though. The yellow w/blk on terminal 'P' has my curiosity up a bit. You stated it goes to port 'U' of bulkhead. So question is why... Is there a wire outside of 'U' and where does it go? horn relay? reverse lamps manual trans only? If there isn't a wire outside of 'U', that female terminal was just a junction point, another wire put in it with the yellow w/blk, branching to something else inside.
I know the 67 horn relay is out by the horns. I don't know what year it moved inside. I know the reverse lamp wiring changed in mid 68, but not sure how much it changed. I know they would sometimes use a bulkhead terminal sort of as, or instead of, a weld splice. No male terminal and wire outside this port.
Its supposed to rain all day tomorrow so maybe I'll study my 67 diagrams and answer my own curiosity/questions. Those answers will lead to more questions. For example... If we find that yellow w/blk does go to the 67 horn relay and that relay was moved inside for 68, got its power from a different source, How do you proceed? Alter this harness or move the 67 relay?