Member of a police unit? SD may mean SicherheitsDienst. NH
To be honest, I am a bit skeptical about it, or at least skeptical that it has any connection to the gas cape bag.
A loose, little slip of paper does not seem to be a very practical property marking for that kind of item. Also, judging from its shape and the stamped number on it, it is of the type that would normally be slipped into a cap's sweat diamond.
Finally, I can't see why a staff member of homeland SD office should be issued a gas cape bag.
So the 56 be is cap size. Might have been part of someone's collection randomly put in. But it was good to find out what it says and where it belong to.