Hello d,
On a 1968 280sl (with the large solenoid)there are a total of nine sockets,there are six right hand and three left hand.
Before you order new ones disconnect the socket from the one on the induction and check the ball for excessive wear,on mine the wear was very bad and i removed the ball and fitted a new one using the ones with the securing nut that are fitted on the cross shaft, part number N 071803 008101.
This is also a good time to remove the pivot/lever that is located under the manifolds to lubricate and change the worn items