A Pratt & Whitney No. 3 with the maker's beautifully-constructed countershaft holding a built-in 2-speed gearbox - similar in some ways to the system used on the Swiss Simonet precision lathe.
If there is one thing that flat belts like, it's plenty of "wrap" onto the pulleys -  which was exactly what the wide, heavy-duty tensioning  system provided on the installation above. More pictures of the gearbox can be seen here.