Cheers Syer.
To be honest this started because after fitting a manifold and cleaning up the subframe i looked down the back of the engine checking everything and saw that nut - however the threads look fresh like something had been bolted in there. If it was unused i would expect them to be dirty/rusty. But I cant see anything that would bolt in there.
I think it might be a mount for another aplication - like the 197/200 torque mount. If that Subframe is used on another car? If it is free it would probably be good for making a gearbox torque link if there is anywhere to attach it to on the gearbox side.