Krieggefangenenpost - it's post for war prisoners
Hm, looks like the Maltese cross.
https://en.wikipedia.org/wiki/Maltese_cross
Also "Die Johanniter" in Germany have a similar logo, they do some voluntary humanitarian work. Saw some ambulances here in Germany.
https://en.wikipedia.org/wiki/Johanniter-Unfall-Hilfe
I'm guessing a prisoner in England.
looks like some labyrinth