I think I found the appropriate sites on the uboatarchiv.
I am still a bit confused, so I ordered a book entitled German U-Boat Losses During World War II: Details of Destruction. I hope this will clarify. I understand that there was confusion between U-879 and U-548 and that that was cleared up. I also read this article: [www.johnchatterton.com]
At the time of that article, apparently the wreck of U-879 had not yet been found. But then I read somewhere that it was found as part of an investigation of the USS Thresher.
This is what I am now hoping to understand fully, whether the wreck of U-879 has been identified. My understanding is that it may have been, but I hope to read more about it.
Thanks so much again for your responses.