I've been flicking through hundreds of old pics of old race minis and I don't understand how cars clearly labelled as a Mk1 Morris Cooper S's have sliding windows and early front aprons?

It is quite easy. The Group Nb Historic class is for cars built up to 31st of December 1964. The Australia Cooper S with wind up windows had not been built yet. The Copper S that is eligible is the English Cooper S with sliding windows.
There is no requirement to use a real 64 UK Cooper S but you have to build it to the same specs as the UK Cooper S.
The early UK 1275 Cooper S was better for racing ad it was lighter in the doors and the dry suspension.

Most of the Minis that raced in Touring Cars in the 60s were UK built cars.
The Series Production cars at Bathurst had to be locally built cars.

this publicity is probably a contributing factor to the old " I had a cooper/s once" story, when it was just an 850.

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Dog Food! Most kids like it more than Broccoli.

It's almost pointless to ask these questions with all the things that used to go on haha

Add no right hand tank to that list too M1100.

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