The Rover 45's were all made locally. As Darryl has suggested, the string of numbers stamped onto the side of the Briggs engine indicate configuration and date of manufacture. The numbers are grouped into 'Model' 'Type' and 'Code'. The first two numbers of the code are the year of the engines manufacture and will give you a rough idea of when the mower was made.
Appearance wise, it's consistent with a late model Rover. It looks like a later model Briggs engine, has the newer style Rover logo on the decals, plastic cover over the clutch assembly and ID sticker on the side of the frame (rather than ID plate riveted to the rear rail).
