Apparently, Hoppy Ate Grape Nuts Flakes Too

Here's a 1951 box of Post Grape=Nuts Flakes with Hopalong Cassidy on the front.

Hoppy!

Here's an early-1950s box of Post Raisin Bran with William (Hopalong Cassidy) Boyd on the front.

Here's A Real Blast From The Past

Here's a theatrical trailer for the first Hopalong Cassidy movie, which, of course, starred William Boyd in the title role.  The original film was released by Paramount Pictures in 1935.