Wow! This piece is rare. I had to search all over the internet to find information.  I pulled this off of a forum- it's a response from the Harmonica museum in Germany about a Harmonica just like this one.

The museum forwarded your inquiry to me. I've not seen that harmonica so I put the question to some of my collector friends. Two of them own Perfectas - the name of the model. Every maker had their own names for harmonicas they produced, usually protected under German copywrite law. Here's a summary of what my friends told me.

Coll.1. My Perfecta is in the original box and has no indication of the maker. The harmonica has tongue protectors [metal bands that cover the dividers between the holes on the wooden body] and says "das instrument das kenners" . It looks like the relic but it is 6" long and has 20 double holes. The box is blue and has the same name and saying on the top with only the key of C on the bottom. I feel sure this was from Klingenthal [a town of harmonica makers in eastern Germany] and was made just before WWII. Since there was virtually no exporting to any of the allies during WWII it was either owned by one of the prisoners or had been lost before the war.

Coll.2. My Perfecta was made by Weidlich [a maker in Klingenthal.] Its says "instrument of the experts." This one seems to have (das instr----- and then kenners) which, according to my German dictionary, means expert. It might be a Weidlich. Mine is about 5-1/8 x 1-1/8 and has tongue protectors on the mouth piece

Stephen, from this input you can be fairly confident your relic was made in the 1930's by a small firm named Weidlich in the town of Klingenthal. It was probably a quality instrument based on the phrase "the instrument of the expert" and also because it had tongue protectors, which commanded a premium price. Based on where you found it, a reasonable conjecture is that it belonged to a POW. (Another translation I found for "kenner" is "connoisseur.")