> As it was probably thought that secret writing would not appeal to girls, the coding/decoding facilities were omitted from the manual.
Whoever made that decision at Mattel was probably male and didn't have daughters or nieces. Just like boys, girls love ways of keeping secrets. I'm remembering the coded notes of my childhood that girls would pass between themselves and boys like me had no hope of deciphering them. Thinking about how my nieces were very happy to learn about things like invisible ink and simple ciphers. One of my niece's favorite gifts was when I bought her a diary with a locking case.
Mattel missed something here. The secret cipher could have been a big selling point for their target audience.