Hello,
All submarine clasps were made of zinc and they usually show some bubbling, like the example below from my collection. Paul, is your clasp made of zinc?
Also, the maker's mark on Paul's example is different than mine. If you look at the mark on the right side of the clasp, mine has a period after 'Ausf.' and there is no period after the 'Ausf' on Paul's clasp. I own a copy of the two volumes series on Kriegsmarine Awards by B&D Publishing. I've looked at all the variations of submarine clasps in those books and they all have a period after the 'Ausf'.
Paul I don't like your clasp but I'm no expert. You might want to post it on Wehrmacht Awards. They have knowledgeable people on Kriegsmarine awards over there.