The lug nuts on a TR6, are not metric. They will respond in a much friendlier manner to a 7/8" socket. Virtually everything else on the car is sized in SAE or unified dimensions as well.
As to the adaptor question, I'm not quite sure what you need to know. What kind of fitting do you reference through which the bolt passes and which holds it in place? We've installed nearly countless TR6 adaptors, and it's a fairly intuitive process. I'm sure you've uncovered a little wrinkle which we can iron out.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon