IMHO that's the BT turret. The pic surface has been cracked at the area giving the illusion that there was an additional line between the rear bottom part and the front side one like it would be welded there. If you have a close up look at the area you may notice the slightly lighter tone of the surface above the line between the front edge and the side observation slot. It indicates the sloped surface going down from the line. The kind of damages to the image surface is quite characteristic for old copies of pictures printed on the photo paper that is more inflexible than the contemporary one. Additionally the shape of the black dirt at the turret back gives the impression that the armour there had the "step" between the turret side surface and the back edge. Just a light trick I would say.