Dan I think the main differences were that the later runs had plastic aprons and the playfield support pegs that have become standard on Stern pros.
Also, some of the last run had spider-webbed rails at the flipper buttons which was a nice bonus.
Mine was one of the last run and I installed the playfield slides for about $50, but the plastic apron does not really matter to me. I was also lucky enough to get one that has the webbed rails.