Hi Pat, and a warm welcome to the forum. It's always great to have another Scott Bonnar owner on board.

Kempe has given you the correct advice re: the missing height adjuster knob.(thanks, Kempe.)
The machine looks as thought it has had a pretty hard life, and can do with lots of TLC.
What I wish to know is if you intend to refurbish or restore it to its former glory, or just get it up and running.
I should warn you at the outset, parts for the Diplomat 430/590 Model Scotties are very thin on the ground as the model was first introduced in 1976, and were discontinued upon the take-over by Rover Australia; consequently very few spare parts survive.
You are in luck if it still has its catcher, as they are like hens-teeth and almost impossible to come by.
If you can supply us with
the complete string of numbers stamped on the metal air cowling of the Briggs engine (Type, Code, etc) we can give you the date of manufacture of the engine, which will then give us the approximate age of your machine.
I can tell you that apart from the missing height knob, it is also missing the engine PTO/engine shaft cover, bit that could be easily fabricated.