Actually, the "confidence" you receive is that you see the oil pump is actually working. In frigid temps back in the era with straight grade oils, it would peg the gauge during initial start up. Once the engine had some heat from idling, the gauge would no longer be pegged and at that point it was safe to begin driving your Model-A. Driving too soon with too thick of oil providing little lubrication and would burn-out the bearings.
As for the Ammeter hole hacked open, didn't these overlay the original instrument panel where the Ignition Switch, Ammeter, and Speedometer attach to that panel?? If so, it needed the clearance for the trim bezel on the Ammeter.
My dad grew up about 50 miles south of Clarinda, IA where these panels were manufactured, when I was 6 or 7 years old, we attended a farm sale just outside of town while we were vacationing at my grandparents. At that sale were 5 NOS Aristocrat panels, -and all but one panel had the gauges still in them. All I remember is there were 4 of one style and 1 of the other. Dad always told the story that he paid $20.00 for the whole lot and sold 4 of them for $100.00 a piece, -which he thought was big money back then. It paid for vacation that summer and then other A parts he needed. I would guess an NOS 1930-31 panel complete with the original Dunn script gauges would likely fetch north of $1500.00 these days.