RobertB

The military required delivery of silver halide microfilm but when you buy copies of microfilm, it will probably be on a cheaper stock which will tend to "wash out" when exposed to the strong light used for scanning. The "loft contours" were usually drawn on metal loft boards so that they would be dimensionally stable. Paper drawings in offices with no air conditioning would shrink or stretch with changes in humidity. Loft contours would be traced from the loft boards onto paper drawings but those were not considered accurate for fabrication. Of course, they're good enough for a model.