That looks like an air conditioning line and tank.

OT: You may want to change your cone filter back to a proper paper one. Paper or oil-bath filters (depending upon which one the car used) are far more efficient at trapping particles that can damage your diesel.

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson