Another C280 engine Question

While replacing the spark plugs in our 94 C280 I noticed when the crossover tube was remove that there is a line with no fitting on it. It is located about the number 5 and 6 fuel injector rail right under the wiring harness. It looks like about a 5/16 or 3/8 id with barbs which to me would indicate a vacuum line of sorts. Cannot not locate anyting else that this open port would be connected to. Any and all ideas welcome, well except for the any UFO theories! Thanks LH
And the car appears to be running well, since plugs and fuel filter change out getting about 24 mpg at about 70+ mph.