From the uboat.net technical page:
The 8,8cm gun
The type VII U-boats had the 8,8cm gun. This weapon is not to be confused with the famous German Army 8,8 anti-tank / anti-aircraft gun which was probably the best weapon of the war, they did not even use the same ammunition. The 8,8cm gun fired a 12-14 kilogram round (9kg warhead), on board were usually 250 rounds. From June 1943 the Atlantic-boats left their bases without the deck gun. Only in the Mediterranean and the Northern Sea boats kept their guns for a few months longer. In July 1944 some of the VIIC boats from the 8th Flotilla in Königsberg got their guns back for the patrols in the Baltic Sea against the Russians.
The 88mm naval deck gun was not capable of anti-aircraft fire since its maximum elevation was only 30 degrees.
It doesn't mention the designation of the weapon.