German 3.7cm fore gun of First World War UB-1 type submarine

Naval Museum, Varna, Bulgaria 2 September 2019

UB 1-17

Salvaged in the Black Sea in 2010. One of this type submarines (the UB-8)  was handed over to the Bulgarian navy in May 1916, renamed Podvodnik No. 18 and after the war ceded to France.