The picture right shows the Commander of U 30, Olt.z. The blue peaked cap was weared from october 1. to april 19. During the war many officers are only wearing the blue cap. The caps for Leutnant z. S., Oberleutnant zur See and Kapit nleutnant.
All badges are silver like that ones of the uniform.
This was a special rank between Kapit n zur See und Admiral. Service-suite of a Marinejustizinspektor.