Update 2:

Once again with some fantastic research from Maciek, he was able to discover more U-boats that were modify for underwater refuelling. In addition Maciek looked at the Type IX’s and discovers several boats that were modify for underwater refuelling. Below is a updated list of all the U-boat that have been confirm by wartime photographs to have been modify for underwater refuelling.

I decided to have a closer look at two of the shipyard U-boat batches. In the table below I have sorted by shipyard batch number and launched date. From this table there is very strong evidence that U-247, U-248, U-311, U-312, U-313, U-314 and U-316 were also modify for underwater refuelling.

U-243 Launched September 1943 F. Krupp Germaniawerft AG, Kiel).
U-244 Launched September 1943 F. Krupp Germaniawerft AG, Kiel).
U-249 (Launched October 1943 from F. Krupp Germaniawerft AG, Kiel).
U-250 (Launched November 1943 from F. Krupp Germaniawerft AG, Kiel).
U-310 Launched January 1943 from Flender Werke AG, Lübeck
U-315 Launched May 1943 from Flender Werke AG, Lübeck
U-317 (Launched September 1943 from Flender Werke AG, Lübeck
U-481 (Launched September 1943 from Deutsche Werke AG, Kiel).
U-766 (Launched May 1943 from Kriegsmarinewerft, Wilhelmshaven).
U-776 (Launched March 1944 from Kriegsmarinewerft, Wilhelmshaven).
U-848 (Type IXD2 Launched October 1943 AG Weser, Bremen)
U-858 (Type IXC/41 Launched September 1943 AG Weser, Bremen)
U-873 (Type IXD2 Launched November 1943 AG Weser, Bremen)
U-883 (Type IXD/42 Launched April 1944 AG Weser, Bremen)
U-1023 (VIIC/41 - Launched May 1944 from Blohm & Voss, Hamburg).
U-1056 (Launched March 1944 F. Krupp Germaniawerft AG, Kiel).
U-1060 (VIIF - Launched March 1943 from F. Krupp Germaniawerft AG, Kiel).
U-1165 (VIIC/41 - Launched July 1943 from Danziger Werft AG, Danzig).
U-1233 (Type IXC/40 Launched December 1943 from Deutsche Werke AG, Kiel).
U-1234 (Type IXC/40 Launched January 1944 from Deutsche Werke AG, Kiel).
U-1229 (Type IXC/41 Launched October 1943 from Deutsche Werke AG, Kiel).