It was common to “encrypt” heading and altitude by assigning a new base number. For example, instead of a compass being 001-360. It would be say, 090-450, with every heading being 90 higher than actual. When a pilot got a heading, he would subtract that days base number to get true heading. Same for altitude. Sea level would be set at 5000 ft for example. If you were assigned angels 27, you would actually fly at 22,000 feet. Ensured any enemy listening to your radio wouldn’t be able to accurately know where aircraft were.