Probably a special customization job in the field, with the top turret locked forward. I don't know if there would be a bombadier (mostly these things were for low level attacks, with all the guns firing forward). Probably removed the bombadier to make room for the 4x50cal in the nose and to save weight. If the top turret was locked in place, also, then it might have been an early version that inspired the solid nose of later models.