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The NAMC’s “Catalog of all Series Matchbox Models” (first edition) does not list any variations not already noted by Nick. Neither does Houghton nor Stannard.
The AIM “1-75 Series Regular Wheels” catalogue (second printing, 1983) has a variation 54-1d listed as 12.5 x 40 BPW, r/a, open one end/closed one end, with aerial and hatch - a plausible wheel/body crossover resulting from stockroom emptying prior to model deletion. If confirmed, this would be Nick's new code 5.
U.K. Matchbox did not cover the model.

An unlisted variation with 12 x 30 BPW (not 12.5 x 30). Otherwise Nick's code 2 - crimped axles, body has one open end and one closed end, with aerial and hatch. This variation could probably have been predicted, as the same 12 mm wheels have been reported on the Maserati 52a.

Nice spotted DrJeep, similar variation is by e.g. 61a, 64a, 67a also
Antonin

Thank you, Antonin. It's nice to find something new, even if it's only a small detail.

I think the models with this shouldered axle are the 52a Maserati, the 54a Saracen, the 61a Ferret and the 67a Saladin. It would be interesting to find out which of them have the 12 mm wheel and how long it lasted. I'll start a new thread in the Regular Wheels forum to see what others think.