Both MK1 S cars I have owned, 66 & 68 builds, had the volt reg located adjacent to the bulkhead and mounted horizontally on its side. They were secured by 2 large self tapper screws and the holes don't appear to have been pre-drilled but it's a bit hard to tell.
The volt reg had to be repositioned as the vacuum hose for the brake booster goes in the area the volt reg would have been mounted if it was located on the inner guard as on other Minis.
The volt reg also interferes with where the fresh air hose would go through the bulkhead hence no fresh air for the S.
UK cars had, IIRC, a 2"?? dia hose with 2 reducers rather than the 3"/3.5"??? dia hose used on later 997 and 998 Coopers. (Early 997 Coopers had a heater fan blower arrangement mounted on the inner guard panel)
I added the 3/3.5" fresh air hose to the 66 car but I had to squash it to get it past the volt reg. At least I had lots of fresh air and didn't need to use the fan.

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68 build Mk1 Cooper S
72 Clubman GT project. Total restoration required

Anyone have a photo / dimensions showing the regulator and fuse box holes on the Aussie Mk1 S.

I don't have a pic of the holes but mine are located here. Car was built in about Dec 1969.