Hi Jack ,
I don't think Whirlwind copied Masport,if you look at the first model Whirlwind 1955 it looks like
Whirlwind were well aware of the Lawn-Boy design, so I think they knew about the offset wheel design
from Lawn-Boy too.

Pics of first model Whirlwind are from Vintagemowers.net

Regards Max.

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whirlwind-18.jpg (104.04 KB, 25 downloads)
whirlwind 18 _1.jpg (91.47 KB, 25 downloads)