Josep,

Gun armament on the U-series diesel-powered torpedo attack boats was very much a moving target. Given that U 63 was commissioned in March 1916, two 88mm guns is a likely initial gun armament. A retrofit to a single 105mm/45 caliber gun or possibly even a 105mm plus a 88mm in late 1963 or early 1917 is quite possible.

The only way to be sure is to have a look at U 63's KTB and see if there are any references to the type of gun(s) fitted. I do have U 63's KTB on microfilm and will have a look the next time I'm at a library with a microfilm reader.

Best wishes,
Michael