If these are like those on the W123, that is to say a concave conical shape with a little hole at its center, I have a grease gun attachment that works great. I'm not sure where I got it, but it is about a 2" long cylinder with a conical point at its tip and a small diameter hole.
Try Northern Tool (
www.northerntool.com) or any major tool outfit like Matco, Snap-On, etc.