I can't say for sure if The Marvel Superheroes Show was ever shown on a Saturday morning anywhere, but I do recall seeing the syndicated series, albeit hazily, airing on a Sunday once upon a time, as filler following football, if memory serves.
Anyway, it was originally produced as a daily series, launching in 1966 on stations across the country. One day, you'd get the Incredible Hulk. The next day, it'd be Iron Man. Sub-Mariner, Captain America, & Thor would round out the rotation.
The last time these cartoons were seen anywhere with any regularity was in the late 70's-early 80's. WPIX in New York had the rights at the time, but rechristened the series, Marvel Men, and edited off the opening and closing credits. Not that we needed to hear any more cheesy theme music than already available, but it would've helped. I have to believe that "Mr. Marvel" himself, Stan Lee, composed the theme songs, too. Describing Sub-Mariner as "neurotic", for example, in the lyrics to his theme sounds like Stan's work for sure. Of these, Iron Man & Hulk would later star in their own separate weekend series, with the others consigned to guest-star gigs whenever possible.
JeffGutman uploaded the open to YouTube. The quality of the video speaks for itself.