Poti-class Corvette
The Poti-class (Pr. 204) were small Anti-Submarine Warfare (ASW) Corvettes built for the Soviet, Romanian, and Bulgarian Navies during the Cold War. They were the first Soviet ships built with Gas Turbine engines. Early models of the type had open mount 57mm guns, with later ones using an enclosed mount instead. The Poti-class were succeeded by Pauk-class Corvettes in Soviet service.
At present, the only way to get the "Early" model of the Poti (with the open-topped gun and RBU-2500s) is through loadouts in the mission editor, not too dissimilar to how you define aircraft loadouts.