Yes, a picture will definitely help. I poured over the manual this afternoon and still cannot determine just what you are describing. The only fuel line that comes close to the area I can think of is the steel line from the fuel distributor to the cold start valve (which also has an electrical connection). The cold start valve is at the lower right corner of the bottom picture I posted.
I did find the vacuum diagram for your car in the manual under the 500SEC files, but it is the same diagram as for the 5.6L engine. There is a vacuum line that crosses through the area on it's way under the intake, but does not connect in that area.
Standing by for pictures (or someone with a 5L)
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.