I believe the first Turbo Compounds were used in the DC-7 and later model Lockheed Constellations. The turbo compound was supposed to deliver better fuel efficiency but initally shortened engine life.
For testing, I believe the first R-3350 Turbo Compound was flown in 1949 in the nose of a B-17. I can not think of any prototypes right now, unless you count the R-3350 TC powered Yak 11 Air Racer.
Here is an article that might answer some of your questions: Turbo Compounds