Thanks for the infoThis is the source:
"Die Ritterkreuzträger der Deutschen Wehrmacht 1939-1945 - Teil IV Die U-Boot-Waffe - Manfred Dörr"
No all I have is the photo of his Verleihungsurkunde on page 34 of Patzwall's book with the rank of Oberleutnant, the date of 18th October 1942 and the wording "Küstenflieger" in the description on it.You have some complementary information aboute "Kapitänleutnant Günter Keller" ?