In going through the pictures in Warship Pictorial #27 Kriegsmarine Type VII U-boats, I found an interesting photo of a (seemingly) VIIC/41 with flooding holes that don't quite match others. Go figure, huh?
Look at the middle boat in the pic below, as per page 68 of the book.
In the upper bow, there are only 9 round holes instead of the usual 12.
In the forward bow, just above the waterline, the 3 oval holes are missing.