Hey everyone thanks for the great and fast replies! A few things I can add now:
I did get the payout mechanism working again, thanks to oldslotman. Actually there were 2 issues, the lever that oldslotman pointed out above, and a dime was stuck at the slide opening of the payout shoot. So that's working again.
This unit has been with my family unchanged for I know the last 50 years so the information everyone provided seems to make sense.
As far as serial numbers go, I could not find anything on the front of the castings at all. I found a sticker on the reel mechanism base that says "Bell Family Mechanism No." I posted a pick of it below.
On the case, I found numbers stamped on the inside, but all appear to be part numbers, not serial numbers, as all numbers start with MLB followed by a number. See the photos listed below. I found those on the reel mechanism base, the base metal plate in the cabinet, the cover plate next to the Jackpot payout window, and the front casting inside of the coin drop box area.
So it looks like I have a Frankenstein unit here
Anyone know what the value might be worth for something like this? And how people go about selling old slots like this?
Thanks again!