Revell 1/72 type VIIIC eduard pe

i have the eduard pe set and am awiting the nautilus wood deck. so far i have drilled/carved out all the ballast vent ports above the waterline. may not do below the waterline since it will be dark gray. any idea what the light box in the gray area on the pic is? i have not seen it in any pictures in any of the 10 u-boat books i now own. also this is my first pe project and probably my first air brush as well.