To clarify things and hopefully simplify, or even over-simplify, the following applies to the classic Mini body-shells.
The Mk.1 & Mk.2 body-shells are virtually identical from a mechanical point of view. There is a slightly larger rear window in the Mk.2, but both have external door hinges, solidly mounted sub-frames, sliding windows and twin-bolt front sub-frame towers.
The Mk.3 has internal door hinges, wind-up windows, twin-bolt front sub-frame towers, doors with squared off bottom lower front corners, larger side windows, different A-panel structure and different B-posts. There a re a lot more detail differences.
From the Mk.3 to the last of the Minis the shells are basically the same except that in around 1976 the front sub-frames were mounted on rubber mountings which meant a slightly different front panel, different exhaust pipe tunnel and large single-bolt front sub-frame tower bolts.
So long as the shell has internal door hinges it is not worth worrying about as any small differences can be sorted out easily if you wish to. Visually they look virtually the same except for the gutter rail on the Mk.3 which is a slightly different construction.
Because Mk.3 cars are all now over 40 years old, anything could have been done and it is not worth asking the DVLA as they won't know and probably won't care.
Just enjoy it for what it is. If you want more info ask a real Mini enthusiast to come & have a good look at your car privately.