Heres how bad they sometimes stamped them, and because the cars were treated and painted afterward they got filled with primer and paint. Mine is so crooked the M and 0 don't show.

It's assumed it was a way of identifying what model the body would become, once the car was painted and assembly began it became less relevant.
In fact on later models they stamped in places that it would be deliberately hidden such as the bonnet rain gutter under a rubber seal or by placing the paint sticker over it.
Another Cooper S check is to look inside the boot up under the parcel shelf.
They glued carpet under felt there on the Cooper S as a sound deadener. Normally it is long gone but you can usually still see bits of old glue and under felt there still. They also added under felt on the floor of the boot and in the boot lid on the S.
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68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)