[Puster Bill]

OK, I got an answer from the NSA historian. Here are the important bits:

The Center for Cryptologic History does not have any raw intercepts of U-Boat messages in its possession. Nor does it have any guide to U-Boat message formats. There is a good possibility that such material resides at the National Archives in College Park, Maryland. There are two records groups where such material might be found: RG 457, The Records of the National Security Agency/Central Security Service, Entry 9032, "Historical Cryptographic Collection." In this collection there are about 12-15 boxes of material dealing with the U-Boat problem. Anothee place may be in RG 38, Records of the Chief of Naval Operations, Commander Naval Security Group. There are several collections within this record group, notably the mistitled "Translations", Entry 1040, which has severl aboxes of material on the U-Boats, some of which are translations, but many boxes hold other material related to the U-Boat problem.

If anyone lives in the area and is familiar with digging around in the National Archives, it would be a greatly appreciated if they could take a look at what is in there.

I haven't heard back from GCHQ yet. I have a couple other avenues I am considering. I'll keep you updated.