Back when distributors had breaker points, that window is where you adjusted them on a GM--with an allen wrench.

IN FACT, before I finally "went" electronic back then, I once shorted a GM dist for my 340. Unfortunately, the , uh, 'machinist' who did the work on the tang got it so far off center, I'd a' been as accurate myself, using a file and a hacksaw. Still got the thing, somewhere