I too have a '76 W115 240D and I just changed the oil out in the ip last weekend for the first time. It is my understanding that some pumps have a small drain tube as you described, and some, like mine, have a banjo bolt on the lower side of the pump that you have to loosen which allows excess oil to drain out the side as you fill the pump with fresh oil from the top. The IP does not get filled to the top of the filler spout.