If you are referring to legal in NBRSA short range, I would say “no”. However, the word “should” makes it seem a bit cloudy. I would feel better if it said “shall or must”.
Stock could be considered legal because there is no convex section. Both the rails and the cutout are flat.
Definition of convex:
convex
[ˌkänˈveks, ˈkänveks]
ADJECTIVE
having an outline or surface curved like the exterior of a circle or sphere.Compare with concave.
"a convex lens"
synonyms:
curved outward · cambered · rounded · bulging · swelling · protuberant · [more]
(of a polygon) having only interior angles measuring less than 180°.