The ring valve on U-637 is possible since the ring valve came briefly into fashion during the end of the war due to its smaller size which made it less likely to be picked up by radar. Rössler does not give details post May 1944, so it is quite possible you are correct that U-637 had a ring valve head. U-637 was taken over by the British at the end of the war. It is interesting to note that the list of snorkel boats on this board does not mention U-637, although there was a U-673 in August 1944.

U-481 was also taken over by the British and sunk on Nov. 30, 1945. If it was carrying a T-valve snorkel, which was the most up to date and still experimental snorkel technology, then it is rather unlikely, but not impossible, the British would have sunk it so quickly. I remain skeptical that U-481 had a T-valve snorkel head. What does the text in Kohl's book say about this? I wish I could see the photo. What page is it on? I will have to wait until I am in Germany again to check it out. Further, the list of snorkel boats on this board tells us that U-481 was equipped in Feb. 1945, 1 month before the type II D, which were supposed to be the first use of the T-valve head were equipped.

The bottom line is there is no conclusive evidence for either U-boat, although I suspect a records mix up with U-637 and a head misidentification for U-481.

Regards,
SuperKraut