The HH at the end didn’t necessarily help crack the code, it was a trick that could be tried once in a while to see if the radio agent on the other end had been compromised. “Heil Hitler” was such an automatic response for German radio operators that they would sometimes automatically “HH” if one was sent to them and they were faking being an allied agent.

Regarding WW2 and cryptography, I Highly recommend Between Silk and Cyanide by Leo Marks. Well written, easy read, dry British humor, and a real view into the British cryptography development, deployment of agents behind enemy lines, and the price that was paid. Can’t recommend it enough.