Refering to your drawing where I have made colour indications (posted below).The pressurehull starts to taper appr. from frame 51, the frames have all identical higths and are circular, following the internal pressurehulls surface (blue), hence the batterycompartmentfloor is following the tapering of pressurehull. Because the boxkeels heel is horizontal, the box keels internal atwortships strengthplates (green) hights are inceasing following the externally the pressurehulltapering. The plates are welded to the outside of the pressurehull exactly where the frames are located inside( green).
Tore