I just installed rubber rings, which I purchased from Marcos. They sent the amount and type of rings that were called out in the manual. What I discovered is that, for my playfield, the amount of rings and the location specified in the manual was incorrect in several instances. I have attached a corrected page "2-56" of the manual, which includes the correct amount of rings, and corrected locations on the playfield.
I don't know if there are different production playfields out there, but this page was corrected for my playfield configuration.