Hi Ozziesim, and a warm welcome to the forum. It's always nice to welcome another Scott Bonnar Model 45 owner on board.
Your machine is a 20" Model 45 (hence the need for the 3rd rear roller)..(ie.) a 3" spacer, and the big brother of the 17" Scotty you are used to seeing.
I can tell from the pic that yours is an early solid deck model...(more collectable) and would have been manufactured in Thebarton, South Australia at the Scott Bonnar factory. It may also have an 8 blade reel as standard, whereas the 17" machine has a 6 blade reel as standard.
Once again Ozziesim,
to OutdoorKing.