A nice set! The text on the back of the card is basically a guarantee and loosely translates as:
'The production of medals and decorations is subject the control of the Prasidialkanzlei. The manufacturer is obligated to exchange unsatisfactorily supplied items against perfect replacement.'
TK posted an LDO boxed EK2 which had a slip of paper with the same block of text.
You can see it here.
Looking for LDO marked EK2s and items relating to U-406.....