Quoted from tcv:Larry: "A marble machine in a jail."
Moe: "That's where they belong."
Hmmmmm... Commentary on gambling? Pun on "marbleheads?"
Very likely alluding to gambling.
Here is another Stooges' short with pinball from 1946, so also pre-flipper. Check around 2:20 and 6:40.
"Look! A game of skill! Shall we play?"