I'm just in the process of making a type of rotisorie for my Rover mini and I noticed 4 threaded holes in the sills. One at each end, left and right. They're 3/8"unc thread. Although I've had heaps of Aust minis, I've no recollection of seeing these. Any idea what theyre for?

It's where it fits to the jig in the factory when the car was made.

Good, 'cause that's where my jig is going to mount. thanks

Lots of them have rusted and weakened, so don't count on them being very strong. They were also for holding the body onto something, may not be suitable for holding a car up upside-down.