On my 1983 300SD the old one had a white, serrated edge, plunger top and you would unscrew the plunger before pumping.

The new one which replaced it (the old one leaked) has a black sprung plunger which you simply push with your thumb. No unscrewing needed. You can see a picture in my maintenance piece on my web site.

As this is a 617 specific question, I would guess there is no difference between the W123 chassis (yours) and the W126 (mine).

Where else could the gasket fit?