An unmarked piece is no drawback but perfectly common and a regular occurrence.
If it was a private purchase piece I would expect to see it marked 'L/58' for Rudolf Souval as that is the maker.
An issue piece would be marked '98' or unmarked. An unmarked piece would indicate an award likely made earlier in the war before the rules changed and all awards had to be marked.
Looking for LDO marked EK2s and items relating to U-406.....
Thank you for that information!