You don't take the doors off, to paint a loco, and you never would remove them, and attempt a lettering job. Usually the letters are a mask, applied, then painted over, although, they can also be decals, or if you have enough money, you could hire a real sign painter, to hand letter the locos. (like we used to do, on the M&E. ) If they are decals, (looking at Scots pic, they are indeed, decals, with the "S" coming from a previous batch, of the wrong size, possibly from another vendor) and they were applied singly, instead of from an applied backing paper, this could result in lettering being applied in an uneven manner. Some folks don't use, or don't know how to use, a chalk line, to align lettering, and logos, when applying them as individual letters. The black paint is straight, across the top hinge line, on the firemans side photo, but the bottom does appear to curve, but looking closely, the catwalk also looks to curve upwards, too. Hard to call, from the angle of the shot. If the painter measured, and masked from the catwalk, and the catwalk IS bowed, that would translate to a curved line in the paint. The top cut line, was obviously following the hinge line, a common technique to aligning masking, or striping. I am NOT saying this is what happened, just what I see, and having participated in the stripping, and painting, of 6 locos, to date, I am familiar with the techniques used, to paint, mask and letter. Regards