I am of the belief that a combination of construction details of the patchbox hinge effect its apparent curvature, which ideally will match the buttstock shape.
1) The pin only effects full, 90 degree opening of the door in the area between the door's outer knuckles. Outside the door knuckles you can bend all you care to with no restrictive effect on door opening.
2) Clearance between the pin and knuckle, an oversize hole within the active knuckle area, allows some curvature of the active area. More clearance, allowing the pin to be more straight line, helps.
3) Some curvature in the active area will correct itself when the door is opened 90 degrees by the finial flexing straighter than the apparent door closed finial/hinge curvature.
4) The hinge knuckle tops may be filed to further enhance the apparent curvature.
Combine all the elements listed and you apparently have a hinge with matching curvature and no evident binding at full opening of the door. I continue to believe the pin will be straight between the door's outer knuckles when fully open. Planning the finial knuckle locations so that the door has a space first rather than a knuckle keeps the active area of the hinge as short as possible.
I listen and watch to learn, and know full well that I am never the final word.
