Had a think and the only way I can think to identify the base is by phone numbers (as stupid as that sounds). When specific capitals went 6-7-8 digits.

Brisbane went 7 digit in 76, so its after 76

These 3 brochures put on by cyberjack provide a better key

Their sydney number was as per Okidokes tag in 1975 https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/85262/rover-pocket-brochure-c1975.html

But in the 1980 and 1985 brochures;https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/95536/rover-rancher-brochure-c1985.html and https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/76144/rover-brochure-1980.html

Both state the sydney number as 6453555.

So by 1980, they were using a different sydney number.

Overall, it leans towards this mower being between a 75 and 79