Don't look at that picture - it's not like the game is doing a snapshot of your current boat and put it there on equipment screen; those pics are prepared by the devs, may be changed by a modder, or not, but are still photos. Salmon/Sargo by default (in TMO) have bow mounted deck guns.
Anyway, I am planning to start another campaign with s-class (actually already did, but was sunk somewhere in Luzon Strait
), but should I receive 4"5 and 3"5 deck guns as a choice (I did)? It wasnt like that in stock game, did it?