This is a theory, not canon and possibly incorrect! But it’s worth consideration. In short were the Zygons originally Zargons from the Gerry Anderson proposed TV series Five Star Five? And if so were any Dinky space models’ designs inspired by this film?
FIVE STAR FIVE -The Movie:
Five Star Five was a live action Gerry Anderson movie project. Gerry Anderson was to executive produce with music producer Sydney Rose. In 1977, immediately after Space: 1999, they began fund raising while long-term collaborator Tony Barwick worked on the script. The story was described as The Magnificent Seven in space. Five characters- including a talking chimp and a robot – must destroy an alien asteroid fortress that threatens a planet.
In 1978 preproduction work began, and Gerry Anderson as co executive Producer, brought in Tony Barwick as Co scriptwriter (Thunderbirds, Captain Scarlet, Joe 90, The Secret Service, UFO and Space:1999), David Lane as Associate Producer, (Thunderbirds, Joe 90,, The Secret Service, UFO, The Investigator and Space:1999) and Keith Wilson as Production Designer,(Thunderbirds, Captain Scarlet, UFO and Space:1999).
Studios were booked, with special effects due to start on 22 October 1979, and live action in Pinewood on 19 November 1979. A few weeks before production began, the project abruptly lost funding and was abandoned.
STORY ARC:
The peaceful planet of Kestra is under threat. The evil Zargon forces are preparing to launch a devastating attack from an asteroid fortress. With the whole Kestran system in the Zargons’ sights, Colonel Zana looks to one man to save them. Except one man isn’t enough. Gathering a crack team around him including a talking chimpanzee, a marauding robot and a mystic monk, John Lovell must infiltrate the enemy base and save Kestra from the Zargons!
Note the spaceship on the bottom left – could this be the inspiration for the 362 Trident Starfighter?
And is the ship top right and middle, the inspiration for the Starcruiser, also designed by Martin Bower, which is strongly linked to the 367 Space Battle cruiser?
Source materials:
https://www.gerryanderson.co.uk/portfolio/five-star-five/
https://catacombs.space1999.net/plus/dt/5star5/vcp5star5.html
Dinky Toys: TV Shows, Space and Specials 