Hi Norm and Ty,
I will share what I have gleaned so far of the Vortex years.
The pic with the late Bob Hawke was one I was thinking of to describe the first appearance of the Vortex in 1983, prior to being released on the market the following year with the introduction of the powertorque engines in the range for 1984.

From a 1985 Victa ad I saw on YouTube, I have gathered that the yellow Vortex came out that year some time, along with the deleting of the hi tech CSIRO licenced blade disc with the stubby blades and replacemed with an adapted ovoid blade carrier with black plastic fans attached to the edges to form an overall round shape to the disc. I believe this was marketed as the "turbo disc" and was included on some other higher end models sold alongside the Vortex such as the white Mustang (second pic) that had the "Turbo Disc" labels on the side.
The green Mustang (first pic) was the first Mustang model with the PT motor and used the same deck casting as the 1970s store brand alloy hi arches. This would have been sold during the red Vortex era before the yellow Vortex and white Mustang took over.
The white Mustang with the same engine dress cowl but now Vortex based deck casting took over some time in 1985 (going by it's brief appearance in the 1985 aforementioned ad) and was positioned just under the yellow Vortex and later the white Mustang GTS (same style cover as Vortex) in price point. From memory of seeing them on display at stores myself, the white Mustang was only slightly cheaper (in the $400 bracket) than the Vortex/ Mustang GTS (in the $500 bracket) and basically offered a model that had the attractive features of the Vortex/white Mustang GTS except with a more conventional cover -at a lower price.
The white Mustang GTS was introduced either late 1986 or during 1987 some time. Who can forget the tv and radio ad campaign?!!
One interesting tidbit I noticed from acquiring different mowers and from observing pics is that some Vortex based alloy high arch PT models like the white Mustang and others had the holes for the Vortex cover plugged up and others had no holes present at all in the casting My one pictured hasn't got the holes. I'm not sure if this signifies an earlier year model or later.

So to sum up, the red Vortex with the much touted large diameter slotted metal blade disc with short blades was sold mainly in 1984 and perhaps into 1985 before the yellow model took over that year with the cheaper "Turbo Disc" plastic finned blade carrier and then the white"rebadged" Mustang GTS replaced that as the final "Vortex" model likely in 1987 and production probably ran for about two years before it ceased for good, even if the base it introduced lived on in other models until 1992 when the steel base shaped alloy deck was introduced.

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Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!