Bill D: You must have grown up in the central San Joaquin Valley. I remember Larry Shehady's Producer's Dairy in Fresno using "Hoppy's Favorite Milk" as their slogan for years. Larry's son, Rich, was a classmate at Fresno State and is now the CEO.

Bill Boyd was a stage and sometime film actor in the 1930's whose career was on the decline. He had been a "matinee idol" type in some early film work, with his striking prematurely silver hair. When the Mulford stories were about to become films produced by Harry Sherman, Boyd was the actor chosen, in 1935. In the books, Cassidy was a rather untidy rascal and rather course in his behavior. However, Boyd chose to play the character as a stalwart "knight of the plains," dressed all in black with silver trim and riding a white horse (Topper).

Boyd, in a daring move for the time, mortgaged everything he had in the late 1940's to buy the film rights for the "new gimmick" of television. He eventually made millions on the franchise, appearing at events all over the country and extolling the virtues of honesty and courage to youngsters. Oddly, Boyd was not especially fond of children in real life, according to those who knew him. Along with Roy Rogers and Gene Autry, he was one of the three biggest western heroes to kids of the 30's, 40's and 50's.

All that said, I don't recall the song either. It must have been associated with one of his earlier films, probably at the time the "singing cowboy" movies were becoming a big deal.