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Airfix military vehicle kits were supposedly 1/76th, which was to fit the railway (railroad to those across the sea) scale of 4 mm / 1 foot. However, the track is 16.5mm gauge, therefore that is undersize anyway (somewhat less that 4' 8.5" at 4mm/1foot), it should be around 18.88mm gauge.
The railway modeller has EM "scale" which uses track of 18mmm which looked better, and more recent P4 (Proto Four) which uses exact scale (1/76th) everything and features suspension on the locos and wagons to ensure they run properly.
The European 'HO' (3.5 mm / 1 foot or 1/87th) is much closer running to scale on 16.5mm gauge track.
The whole matter is further confused because railway model manufacturers often adjusted the width or height of their models to fit in with the rest of their range.
It must also be remembered that these were essentially toy companies, so scale was not as important as having something that "worked".
I think Airfix used 'OO/HO' as a marketing tool, though it certainly led to much confusion over the years.
Incidentally, a few narrow gauge railway modellers have adopted 1/35th as a scale for their models in recent years.