On a whim, the other week, I stopped at a CVS Pharmacy and snagged a copy of The Ultimate Underdog Collection, Volume 1 on DVD, collecting six episodes from the series, backup features included.
Each chapter to a 4-part Underdog serial, stretched across two episodes, runs about 4-6 minutes each, shorter than the running time of the average short of the period. That does leave room for 2 backups, usually reruns from earlier series from the same studio, including King Leonardo & His Short Subjects and Tennessee Tuxedo & His Tales. The World of Commander McBragg has different title cards than we became accustomed to via syndicated reruns, but the intros to each of the backups, including Go Go Gophers, are not included in this set.
Right now, let's scope out the first half of "Fearo", plus Go Go Gophers & The World of Commander McBragg:
You'll note that the clip used to set up the first Underdog segment of each show was usually used later in syndicated reruns. Go figure.