they do have little bit of a curvature

Toine,

I have two pairs of MkIIIa's and a pair of MkIII's I have had the lenses out of all of them and they all show to some degree a curvature. The ones in the 38 and 39 dated RAF mkIIIa's are curved more than the ones in my 42 dated RCAF MkIII's. These lenses are quite thick so if using a replacement of equal thickness I would think they would need to be shaped to the frame. In my opinion not shaping them would run the risk of damaging the frames.

Interestingly there is also a curved very thin blackend brass fillet that comes out with the lens, I wonder if it is there to help keep the lenses in shape?