In the past, I have been less than polite to Walter on the Chinook thread, however, it's a genuine question that deserves an answer without abuse........ Until he ignores genuine answers of course.
Most aircraft require the compass to be calibrated at regular intervals. The adjustments can never be perfect, so a small card is marked with "deviations" or inaccuracies to help the pilot with exact heading measurement. In reality, the one or two degree adjustments are beyond most sky gods, meaning the card is next to useless. It is an airworthiness requirement.
The card has to go somewhere in the cluttered cockpit, in the EC135 it sits on the central windscreen post. There isn't anything behind it.