Ahhh, that is where you insert the coins. Then, and you can’t really see this, there is a black bowtie at the base of the duck’s breast that you turn, which activates the gumball dispensing mechanism beneath the globe. When you finish turning the bowtie, a gumball (or marble in this case) drops out of the duck’s, uh, ass.
testing 
I want one of those.
Found it in a pawn shop on Greenville Avenue in Dallas in 1991. Never seen another since.
What’s the slot in the neck?
Ahhh, that is where you insert the coins. Then, and you can’t really see this, there is a black bowtie at the base of the duck’s breast that you turn, which activates the gumball dispensing mechanism beneath the globe. When you finish turning the bowtie, a gumball (or marble in this case) drops out of the duck’s, uh, ass.