Here's the reply I got -

Dear Sir,

yes you are right in your assumption that the Me 109 aft fuselage is actually a poly-edged shape and not a continuous curved line. The sections are deep drawn ( stress formed over forming blocks ) and have a fwd and an aft flange worked out of the skin. These "ribbed" sections are joined by sections rolled from flat-stock sheets...
Actually ( if you look on top of the fuselage ) the fuselage widens or narrows in steps at each joint between these sections.

Best regards

Claus Colling

So now we know the 109's aft fuselage is, as I suspected, straight sections. Makes 3D modelling of it much easier, and it means I can make a plug from cross sections only and cover it with developed litho plate panels. I suspect if you made one with straight sections and one with compund curvature and put them side by side you'd probably not pick them apart if you didn't know.