Roy Rogers and Post Cereals had a licensing agreement for an unknown period of time, but it at least covered the period between 1952-1955 (I’m sure the Roy Rogers & Dale Evans archives can better answer the time period question). Together, the entertainer and cereal manufacturer produced a series of TV spots, print ads, and merchandise.
Here’s an example of a Post Cereal Commercial with Roy Rogers, Pat Brady and a hint of Nelly Belle (see many more commercials here):
In 1952 and 1953, Roy Rogers teamed up with Post Cereals to produce a series of 36 pop-out cards about Roy’s epic adventures. View references to the names for the entire list of cards here.

Card #4 “Pat Brady and Hi Jeep, “Nelly Bell”. Of course, the front is actually the same size as the back. See better images here.
Post Cereals also released a series of 3D cards with Roy Rogers, a few that included jeeps, that required the use of 3D glasses. Here are two examples that include jeeps (there are some of these 3D items for sale on eBay).
And one more off of eBay:
In 1953 Post Cereals joined Roy for a series of pins, one of which included NellyBelle on it.