Hi to All,
I have had the pleasure of talking to a gentleman today, who owned his own business; part of which was selling and repairing, sharpening, and spare parts for all models of Scott Bonnar reel mowers and has kept excellent records.
From his records, we can now state that the 
 Model 17 Supercut was first released in 1950,the 
Bonmow in 1953, the 
Model 33 in 1960 , and the 
Model 45 was first released in 1968 (with the Kirby-Tecumseh engine)and 
Model 45 with the 2.5HP Briggs (Model 450520 17") in late1973/1974 which later became the Model 450589 17" with the larger Briggs 3HP. engine; prior to them being badged Rover-Scott Bonnar after the take over in 1980 and finally as Rover in 1983. 
The difference in the model numbers sometimes relates to the width of cut (eg. 14", 17", 18", 20" etc.) and also changes in engine manufacturers, the engine size (also petrol or electric and also battery powered).  
 This information has proved invaluable as we can now date our machines a little bit more accurately, though, unfortunately not an exact date. But at least we're in the ballpark.