Perhaps they had window material the same thickness as the frame, and wanted the windows to be flush. In that event, a 1/16" wide ledge of transparent styrene wouldn't show, very much.

I did something like that, putting flush windows in a BUSA 25% Cub. Turned out nice, you can see the backup pieces if you look closely. The glazing is nice and flush.

I would think the shape of the rear side windows on the P40 would make the flush window approach pretty labor intense.

Best wishes,
Dave Olson