More likely, that cel or drawing you bought was from a TV commercial, either produced contemporaneously with the original series or something more recent. Below is an example from eBay of a cel from a vintage early 1960s Cheerios commercial signed by director Bill Hurtz, writer/storyboard artist Chris Jenkyns and the late June Foray, voice of Rocky the Flying Squirrel. Should you wish to purchase this cel, you can buy it here.
For some time now, more recent drawings and cels of both Rocky and Bullwinkle have been offered for sale and are often misleadingly labeled so that buyers might believe they were buying vintage art; kudos to those dealers who have accurately described what they were selling. The art in question likely came from a 1986 commercial for Hershey's Kisses. The commercial can be viewed below:
Almost looks vintage, doesn't it? If the print were faded, one might almost mistake it for an early 1960s ad. However, it was produced in 1986 at Filmfair, a highly regarded Los Angeles commercial studio, because Jay Ward had stopped producing commercials in-house in 1984. The spot was notable for several reasons, not the least of which was it was Bill Scott's last performance as Bullwinkle. Building on that foundation, it was laid out and directed by Sam Cornell, a Jay Ward alum from the George of the Jungle series. (You can read more about Sam on pages 277 and 305 in The Art of Jay Ward.) Here, for sale on eBay, is one of Sam's layout drawings from the commercial:
Adding to the spot's luster, it was animated by Chris Buck who had left Disney a couple of years earlier after a 6 year run. Chris was working freelance for Sam Cornell and Filmfair on commercials during the mid-80s and later returned to Disney several times, serving as a key animator on many of the 80s and 90s hand drawn features and later as co-director of both Tarzan and Disney's mega-hit Frozen. In between stints at Disney, he directed the TV series, Family Dog and later directed the Sony feature, Surf's Up!
The character models in the Hershey commercial are spot on and, adding to the vintage look, the cels were hand inked. The image below was slated to be included in The Art of Jay Ward Productions but unfortunately it could not be located before the book went to press. If you bought art from the above commercial, it might not be vintage but it still has a strong pedigree!