Yep, what Chegny said about the round barrel. The idea on that was, so techs could just put a probe in that hole and it would read the DASH line on the balancer. IIRC, there were even some models that came out and that's all they had. No pointer or numbers on it at all. At any rate, that was when Scope tuning was real big. Kinda back in the days when I was starting to work on cars and do CA Smog Emmisions testing. You just hooked up all your leads and probes, and then read all that **** and make your adjustments as needed.
I remember too, those were the days of the $39.95 Quick Tune. Hook it all up, read the scope. There's 2 plugs not doing so good, 1 plug wire not in the best of shape. Replace the 2 plugs, replace 1 plug wire, re run the scope, all good. DONE !!! Collect $39.95 and see them next week for another $39.95 and this time replace 2 different plug wires and maybe another plug.
No Joke. That's the way it was done. It was common to need a tune up about evrey 3 months if you did the Jiffy Lube & Tune.