Ok so still don’t have this right.
there is clearly a braided vac line on the one port I see on the side of the distributor. It starts right behind the air box. I believe this is the advance line.
My mystery line starts kind of behind that one. It attaches to a T junction and two other vac lines. I was looking at the diagrams on this site:
https://nineelevenheaven.wordpress.com/vacuum-connections-on-the-k-jetronic-for-the-911-sc/
Could I be looking at an unused cruise control line (green)? I didn’t think my car had cruise control optioned but if it did it was removed by a p.o. Cruise control was not listed on the spec sheet I got from Porsche recently.