What is the earliest reference to the game Bulls and Cows?
According to codebreaker-mastermind-superhirn.blogspot.co.uk
The exact year when the number guessing game Bulls and Cows was invented is not known. "Bulls and Cows has been played as a paper-and-pencil game for a century or more. I first played a computer version in 1968 on Titan, the Cambridge University Atlas" (John Francis, 2010 [dead link to "Bulls and cows.pdf"]).
A similar word guessing game "Jotto was invented in 1955 by Morton M. Rosenfeld and marketed by his New York-based Jotto Corp." (wikipedia.org)