I have noticed that that particular Impala 2 is so far from the original shape.
It appears that the only original parts of the mower is the base and dress cowl.That one now has a 160cc engine on it (It should be a 125cc),(also as pointed out) now has a G4 or LM carby not the original G3. Also the handles are from a series 80's mower and I'm assuming must have had holes drilled into it to put the bolts through for them.
As for the other things mentioned the Impala 2 did in-fact have a short release time,from 1972 to 1973.Where as its cousin the VC-125 Mark III was from 1973 to 1974.The bases between the two models were also very different.So much so I can't work out why they would go to the trouble of making a different cast base for a model that wasn't used for very long.The Impala 4 used the same base as the Impala 2 but had a larger hole for the engine.As far as rareness goes they are both very very hard to find it has taken me a very long time to find both models,but yes I would agree with Jack the Impala 2 is a little harder to find.It's just a shame that the one on eBay is so far from it's original state other wise I would have been all over it like fly's.....well you know the rest of it.
Here is a picture of mine Impala 2 before I pulled it down to start restoring it,and the bottom 3 are of one of my VC-125 mark III's.If you look at the bases you should be able to see some differences in the casting of the bases.
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Here for a good time,not a long time.