I can't believe I missed this topic first time around, being as I'm a big B-Western fan...even preparing a proposal for a class on the genre (not to be taught by me).

There is a bit of misinformation about the films posted back then, none serious, but for the record.

The series is not considered a "trio series" in the accepted meaning of that term. They are Hoppy films. William Boyd as Hoppy always got top billing, with his sidekicks getting lesser billing. The Hoppy character was clearly the boss (usually the foreman). In the typical trio series--The Three Mesquiteers being the longest running--the characters were basically co-equals, as was the billing. Often two were romantic rivals, and the third was somewhat of a comic figure.

Regarding Robert Mitchum: he did appear in about a half dozen Hoppys in 1943, the first being Hoppy Serves a Writ. But his first confirmed film role--by mere days according to release figures--was The Human Comedy, a very touching film of WWII on the homefront. Mitchum, by the way, was slated to replace Tim Holt in the Zane Grey western series at RKO, but his fine notices in The Story of GI Joe rescued him from the oblivion of the B-Western.

Other well known actors who early played in Hoppy films were George (Superman) Reeves, Duncan (Cisco Kid) Renaldo and Lee J. Cobb (once billed as Lee Colt!).

I don't know if anyone mentioned it above, but running concurrent with the TV half hours there was a a Hopalong Cassidy weekly radio show.