D6H vs D6H Series 2

Hello all, first post here. 
I'm in the market for a D6H/D6R class machine.
My question is: What are the technical differences in between the D6H base model that was first offered in the 80s, and those later models that carried the "Series II" description toward the end of that product line production?
I've read that there were steering pump improvements and slight engineering improvements over the original. Is that true? School me.
Thanks for your time!
-Nick