This cartoon is basically just a play on words. You see “Founded” on signs a lot followed by a year, so adding “Dumbfounded” and some additional years isn’t a huge stretch. The trick for me on this one was, as usual, the details.
Which years would be dumbfounding? Was it early on in the business? Some sort of later issue? How long did it last? Which years sound funniest? Should it be an even or odd number of years?
I wish I could say there’s some sort of cartoon calculus to offer some answers, but mostly you just feel it out. (Which is good because I’m bad at math.)