Mort Walker’s Beetle Bailey for the 13th of August, 1964, reprinted the 1st of March, 2021. Also this past month I learned that Sergeant Snorkel’s look was based on an actual person Mort Walker trained under, First Sergeant Octavian N Savu, who apparently never knew anything about it himself. The Chaplain’s name is Stainglass, by the way. I have no information about the squirrel’s name or real-world inspiration.
So, yes, we can say that Mort Walker had seen squirrels in 1964.