The mower appears to be a Colt 5, Model 72721, with an 11 cubic inch, second design generation, vertical crankshaft Briggs and Stratton engine with a Pulsa-Jet carburetor, plain main bearings and a rewind starter. The engine was manufactured on 7 October, 1981. (The mower was probably made a few months later than that.) I have no idea why it had Scott Bonnar badging as well as Rover, but SB was a subsidiary of Rover by then.
