In the SDK there's actually an unused Slot Machine which was supposedly going to be functional. It had animations and sound effects which can be seen in this video:
Killing Floor Unused Slot Machine - YouTube
In the game itself the slot reels have icons of Dosh, Dosh x6, an icon of a shield, an icon of a knife and gun, a Jackpot icon, and an icon of Pukey the clown himself. From the sound effects along with the coding and animations it can be found that the slot machine would have either:
Pukey Icon - Puke on the player
Dosh Icon - Spawn 1000 cash for the player
Dosh x6 Icon - Spawn 1000 cash for the team
Jackpot Icon - Destroy the machine
Armor Icon - Spawn armor
Weapon Icon - Spawn a random weapon
The weapon would pop out of the slot machine and only the player betting on the machine would be able to see it.
The unused achievement icon (as seen on the right) looks as if the clown is puking. Possibly leading to an unused slot machine achievement.
Now this is just my own speculation. Heck, I could be completely off. But this is just what I think. Was still pretty interesting to find. It seems like it would be an OP interactive prop though, so I can see why it was never included.
Good eye though, alienM.