Big Gun Subs

[Letum]

From The London Treaty:

Each of the High Contracting Parties may, however, retain, build or acquire a maximum number of three submarines of a standard displacement not exceeding 2,800 tons (2,845 metric tons); these submarines may carry guns not above 6.1 inch (155 mm) calibre. Within this number, France may retain one unit, already launched, of 2,880 tons (2,926 metric tons), with guns the calibre of which is 8 inches (203 mm).

So France had a U-Boat with 8 Inch guns shortly after WW1 ?
Blimey!

Did big Gun Subs ever work in WWI or in the inter-war years? Are there any impressive examples?
The Germans had planns for a Type XI in 1937. The boat had 4 X 12.7cm Guns but was never built. Or was it....? http://www.uboatwar.net/XI.htm
(highly unlikely!)


Here is a artists impression of the Type IX: