At this stage I think Dembonees mower is the 1984 Australian version Roper ,model number L1904BR with the 3 forward gears plus reverse with the high /low shift giving 8 different ratios .
Seventeen and a half inch blades multiplied by two equals 35 inch plus some clearance would make the deck 36 inches.
The American equivalent would be the Roper L121 1979 to 1981. http://www.tractordata.com/lawn-tractors/003/5/9/3595-roper-l121.html
The use of 3 blades were on higher HP mowers with the Roper mower,see pic below.
A quick newspaper search ,only found the 1984 Roper Rally LO81
Dembones Roper Mower looks to be the same as one on ODK. https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/21163/anyone-heard-of-rally-roper.html
YouTube Video showing location of model number. https://www.youtube.com/watch?v=_VX9WzMObts
