Hi Agrid,
Well the engine was assembled on the 25th of March 1977 and with that date it pretty much looks to be the original engine fitted, thus the mower would have been assembled in the Thebarton factory in Adelaide around July or August of that same year. The building of the mower chassis's themselves is not possible to an date exactly as Scott Bonnar records were all destroyed when Rover took them over in 1980.
All in all it looks to be a fairly good mower.
Cheers,
BB