According to Wikipedia, Knock-Knock jokes were first made popular in 1936 on Fred Allen's radio show. While reading a copy of "Police Comics" from 1941 I found this sequence that's essentially a visual version of a knock-knock joke and I wondered how long the artist had been planning to use it. Because of the layout of the comic the first two panels are in the middle of the page, while the second two show at the bottom. I took the liberty of laying it out as four consecutive panels, the way you might see it in the daily comics.
The character is #711, a man who was wrongfully sent to prison for a crime he didn't commit. As part of his crusade for justice he puts on a cape and hat and tucks his pants into his socks (yes!), and escapes prison on a nightly basis to fight criminals and right wrongs. He took his name from his prisoner number, so it seems odd that the warden doesn't suspect a thing. The comic fails to explain why his normally green prison garb turns maroon when he's in costume as #711.