Wow, that's a trip down memory lane... Goldberg Stunt Man 23 with a Golden Bee .049 was one of my first control liners when I was a kid, later put a lot of miles on a Goldberg Wittman Buster with a Fox .35 (speaking of wedge tanks strapped on the side of a profile fuselage).
As for the Tee Dee .051, I not only had one, I know why Cox made them. You could put a Tee Dee .049 on your free flight duration plane and enter it in the 1/2A class at a contest, then swap the engine with an .051 and fly it in the A class at the same meet.
Good times.