Never saw this air cleaner before...

I bet if you take the air cleaner off you will find a vacuum draw propane regulator on it. The fitting connects to the vaporizer/primary regulator. The flow of propane is controlled by throttle position - on the carb. The vaporizer/final regulator lets the engine pull the propane into the engine through the carb and the gas is shut off when the propane is running. The real trick is switching back to gasoline after running on propane. You have to shut BOTH fuels off until it coughs and then turn the gas on real quick. There is always residual propane after you shut the valve off so you have to run it out or the engine will be so rich it will die.

 

Looks like some sort of mutation growing out of the carb..