Mill's Wolf Head front casting

[Frank]

There is a removable panel in my front casting - about 2" by 6" - what is that for?

[Midcoast Vintage]

it's called a "jackpot inspection plate".
it is so the operator could line up the jackpot trip with the jackpot if not working correctly, and, to adjust /re-align if he had to switch out a mechanism.
it saved a lot of time, as, the operator / mechanic would not have to take the mech in and out a jillion times to do adjustments.