The disc thickness isn’t mentioned on the extension referring to the large diameter discs, which as stated above, were able to be used with 15” wheels from July 1974. As I have 304mm diameter on my car I know they are 25mm.....
I’d just be a little concerned that if 260 x 25mm wasn’t used in period, it would prohibit it’s use now- BUT as the Monte Carlo caliper is commonly used with small discs and 13” wheels, I guess it must be acceptable!
