How many of the Original 1958 100+ Exist

As far as I know, the term 'F' head is a purely American one. Overhead inlet, side exhaust engines were very common in the early uears of motoring, where this layout (and T or L side valve) allowed easy access to valves in an engine with non-detachable heads.

The Rover setup, however, is so different from this early setup that it is not certain that it really an appropriate use of the term. Rover used a variation of that introduced about 1930 by Bentley, and the inclined top to the block with the combustion chamber entirely in the block and piston enables a very nearly hemispherical combustion chamber, with a large intake valve on the flat surface not having the size limited by the small bore dictated by the UK horsepower and tax rules.

Compare for example the Willys effort, which, unlike Rover, was a simple conversion of a side valve engine to IOE, by providing a new head with overhead inlet valves. This enabled a higher compression ratio by making the combustion chamber smaller, and larger intake and exhaust valves, but still with a not very attractive combustion chamber shape.

John