Dave,

Here is a picture (the best I could do at the moment) of the pistons, the one on top is on the 175 pump, 170 on thebottom, it has a taper on top and the lubrication groves are bigger , I guess with better lubrication it should last longer.
The pump internally pretty much remains the same.

I hope this helps and is what you wanted to see.

Regards

Alejandro