According to the Free Dictionary, citing the McGraw-Hill Dictionary of Scientific & Technical Terms, a "stunt box" is "A device to control the nonprinting functions of a teletypewriter terminal." This answer here also mentions it using that meaning.
The CDC 6600 supercomputer also used a "stunt box" to handle transferring data between the many interleaved banks of RAM ("central storage") and the CPU. (Thornton, Design of a Computer: The Control Data 6600, p. 16.) It would handle things like holding a request that arrived while a memory bank was busy and delivering the request to the bank's memory controller once it became free.
How did the designers of each of these devices arrive at the name "stunt box"? Are there other devices also named "stunt box"? Is there some sort of similarity between these different devices that makes using this name make sense?