I talked to a route operator back around 1980, and I asked him about these scratched out serial numbers that were mostly on Gottlieb machines. He told me if a distributor was an "exclusive distributor" and wanted to sell machines made by another manufacturer, the distributor would need to secretly buy these unauthorized machines. So, if exclusive Bally distributors wanted to sell Gottlieb machines, they would need to obtain the Gottlieb machines through some source - perhaps a rouge Gottlieb distributor or directly from the factory. To prevent Bally from finding out that "their" distributors were also selling Gottlieb machines, the distributors would scratch out the serial numbers on all the Gottlieb machines so they couldn't be traced... I'm unsure if this is true, but it's an interesting story...