Further to more info, I've found that the original carb has not got a broken screw after all. It didn't have one to start with. Being a later model carb it is a fixed jet. All to do with emissions. It still has the wrong float bowl though.
I checked the float settings on both carbs. The older carb has a brass float, the later one plastic. I set the height of the brass float but was unable to set the drop. The tag, as in Chris's diagram, is missing.
The plastic float, much bigger than the brass one, was ok for both settings. Unfortunately they are not interchangeable and neither are the float needles.
The older carb is now all together and on the tractor, ready to try tomorrow.
One thing for sure is I'm learning about the carbs.