The Tweet included an image of the Nazi zombie used in the reveal art which has been altered to make the hidden code appear, and also the letters of the code with a yellow outline around the location on the helmet of the Nazi zombie.
The earliest account of this secret cipher code that I could find came from a tweet from the well-known gaming Vlogger, Ali-A and also a later Vlog which described the same and thanked another Twitter user named only as Luke, who apparently first pointed this cipher out to Ali-A.
