I'll second the training session idea for two reasons: all the machine manuals lying on the tables and the opened-up games in the background as mentioned. Probably a repair tech event. A few of the people look familiar, but I wasn't taking photos at pinball events that far back so I have none to compare with. The one I think I can identify is the kid with the "10" shirt: I believe that's Dallas Overturf, who beat me for the US Open title in 1980. I'll pass the photo along to a friend of his who might be able to confirm it.