Hello RoRo
I feel the 'SER' is a red herring ...
I think it's 'SER.' - an abbreviation of 'serial number'
The USA produced, I guess, dozens of these 'Big Wheel' mowers,
some - like this one - a copy of the Yazoo drive system.
These Big Wheel designs never caught on in Australia, though
the Yazoo was imported.
Your machine's Tecumseh indicates to me this is a 1970s machine
(possibly a bit later). I think a North American forum might
reveal identification - but I do not think these were sold in AUS.
Hope this helps.
-------------------------
Jack