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"..he has replied quickly, and sent a pic of a fully restored Colda, painted emerald green deck...there is no decal on front, of course"
Hello BH
Yes, I use to write for that small forum.
That particular machine was presented in this post in un-restored form.
It was No.2 to your No.3 machine.
[Linked Image]

The restoration colours do not accord with the evidence in the original un-restored photo.
Quite a loss (in my view).

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"the simplex in those pics looks a bit like mine, too, doesn't it."
Not really BH. I have written about that important lawnmower here:-
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=51528

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"surprising there have even been this many local mower manufacturers, frankly, must have been a thriving market..."
I have begun to write about how our lawnmower industry started in
the 1930s - brought about by protective tariffs. This takes noting away
from a couple of pioneers that started prior to 1930, the most important
being Scott Bonnar.

The rotary revolution of the late 1940s and 1950s produced a few dozen
manufacturers. Most were small firms, who could eke out a living because
of the simpler designs of the time.

All that changed by the start of the sixties. Our expectations changed
about what a lawnmower should be. Only half a dozen 'big players'
could survive in that climate.

Colda was not a small firm. In this late 1956 advert we see the size
of the factory. Note that this is the year before their lawnmower would
be made. Colda had clear capacity to deep press bases with a high skirt
(for cutting efficiency).
[Linked Image]

I have argued here that your machine is more probable than not a Colda.
It is not yet conclusive. That would require an illustrated advert or brochure.

I have argued that what killed off the Colda was probably the Malley's takeover.
The Colda design (like yours) was too old for the 1960s and considerable
capital would have been needed to produce new designs. I might also add that
lawnmower manufacturers in the 1960s needed a 'range', not just one design.

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"Now, what, I don't think there are any local-powered local brands, are there? Victa use imported engines various brands, i think the Victa 160cc 2-stroke is officially extinct, isn't it? And Rover always did mostly just use Briggs 4-strokes."
That's right. Victa have signalled their intention to 'phase out' the 2-stroke
over the next few years.

Rover (first named Wizard) started out using the Australian-made Simplex engine.
They then used Villiers, Kirby, Briggs, Suzuki, and Tecumseh engines.
It is true that by the late sixties, Briggs was their preferred power unit.

Hope this helps.
-------------------------
Jack