Jim,
It does indeed sound like this car is a "mutt". That is not so unusual in North America for minis. I've seen many many backdated, forward dated, and maybe even sideways dated cars! Many had real mixed and matched equipment. Disc brakes on the front would certainly be a plus, but I have a couple of customers with stock 1275 motors in drum braked cars, and they don't seem so bad. What OZ has to keep in mind is that his is an ANIMAL!!! It could use SIX WHEEL DISC BRAKES!!
Also, if you had a stock '67 or '71 Midget, I assure you they each had disc brakes on the front. I've NEVER seen anyone backdate the brakes on one of those, and it's seriously unlikely.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon