I have owned both Bumper and Skipper and was fortunate to have all the considerable amount of paperwork that Bally shipped to the original operator for both these machines. I still have copes of this paperwork although it has taken me some time to locate these among all my printed material.
Included in these these is a full page of instructions for Bumper entitled "Free Games Adjustment" another sheet entitled "Instructions To Merchant (i.e. the operator) concerning the Free Games Meter and a number of Instruction cards to use on the machine. All of these are clearly marked Bumper and item number in small print at the left bottom corner. I will scan these items and add them to this posting. There are also sheets dealing with Adjustment of Score Projector, Operating Instructions to set up the machine, Adjustments of Contacts in Lightbox as well as a Wiring Diagram.
Playing or owning a machine does not always mean one understands its operation fully. In my time I have come across some quite knowledgeable people who were not aware of their machine's complexities. I owned a Bally Mazooma Bell "flasher" for many years until I was able to find a Manual and schematic for it and only then did I understand some of its workings.