U-Boat disk?

[Eike41]

I have this KM disk marked 4523 under the letter O. I have been told it is a U-boat disk but I cannot find any reference material on that and I don't read German well enough to understand what I have found.

Any insight would be appreciated.

[meskalin]

This is a normal Navy brand.
They are called on the basis of stamping "old style".
THe 4 digit number is the personal number, the "O" stands for Baltic Sea (Ostsee).