The IMDb Goofs section says a Reliant Robin is visible in one shot and apparently this is a mistake as this model wasn't introduced until 1979.
However the film was released in 1976!
Certainly Reliant Robins weren't around in WWII but it is also certain that a time machine wasn't in operation on the set that brought this Robin back from the future specially for that one shot.
I could be a Reliant Regal the Robins predecssor for some reason people call all of Reliants cars Robins.
Like in Only fools and horses Del Boy drives a Reliant Regal Supervan 3
but people call it a Robin
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There were Reliant three wheeler motor vehicles around during the second world war - in fact they went back to the thirties.
I did not notice the goof, so am not suggesting what was captured on film was correct for the era, but depending on the model it could possibly have been.
http://danlockton.co.uk/rebel/early_years.html
from imdb When the American Rangers are travelling towards the church where the villagers are being held, there is a Reliant Regal in a farm yard on the right hand side of the image - this model was only introduced in 1953.
(https://www.imdb.com/title/tt0074452/goofs)