Here's a thread with pictures of the various pumps and the Stihl numbers -
http://www.arboristsite.com/chainsaw/242389.htm
I just popped in the upgraded piston rod on my old pump. It changed the stroke distance from .4mm to .88mm.
The newer pumps have what appears to be a piston on top of the rod. It is not a 'bearing', it is a piston to keep the volume of the cam chamber the same as the piston rod moves. You can pack the cam chamber with grease and not have to worry about it getting pumped out.
