I've been playing pinball for 30 years now and one thing that I've notice is that for decades (pre-1990), the start button on pinball machines was never labeled. Everytime I went to the arcade in the past, there would always be at least one person inserting a coin in but was confused on why the game wasn't starting. I also fell victim to it on the first pinball machine that I've ever played (Raven). It was in a dark rollerskating rank and for some reason the start button was black. Luckily one of my friends pointed it out to me, that the non-labeled black button in the front was the start button. Keep in mind, that this was on my 8th birthday and I played lots of redemption machines (claw machines, Skee-ball, Whack-a-Mole) and Konami arcade machines (TMNT, Simpons, etc...) prior to this. These machines would start up just after inserting a coin in. If they did have a start button, it was properly labeled or it would light up.
Looking back at it, I'm actually dumbfounded at how long it took for the pinball industry to start labeling them. Williams started labeling them with their System 11c tables. The second machine that I've ever played (Rollergames) had the word "start" painted on the cabin above the button. A year later and every manufacturer started to use light ups, which completely resolved the confussion. At the time, pinball has been around for over 50 years. What took them so long in applying such a simple concept? I guess we will never know.