This is a 1/350th scale after deckhouse and D shaped gun tub for the Trumpeter or Tamiya 1/350th USS Fletcher-class model. This is meant to represent the type applied by Mare Island to refiting Fletchers. Check your references to ensure that this is the correct part for your model as there were many different versions each with little detail differences.
This version represents a tub with a Mark 51 director (not included). The mounting hole for the aftermarket, modeler supplied 40mm Bofors is 1 mm in diameter. Apply your favorite aftermarket photoetch doors to the door outline, or you can leave it as is. Your choice.
Clean the part by soaking it in lukewarm soapy water and then CAREFULLY scrubbing it with a soft toothbrush or similar. Be carefully, as the splinter shielding is just 0.3 mm thin and is easily damaged. Acrylic modeling paints are probably best. Consider painting the interior of the part, as well, to reduce light transfer through the otherwise mostly clear part. This will reduce the need to overpaint the exterior, allowing the parts finer details to not be covered over with paint.