difference between zip 2 stroke carbs

what is the difference between a zip carb on a 2 sroke aircooled zip and water cooled are they interchangeable

 

There are both the same unless one of the bikes they come from was a old type (rare) then it would be a 12mm not a 17.5mm, but if there both 17.5mm they are fully interchangeable as there the same the only diffrence is the water cooled one will have 2 nipples on for the carb heater

 

i have the 12mm one fitted now as its a mk1 and its near on impossible to buy jets for it thats why i am after a 17.5 carb