X-Men: Days Of Future Past Includes Nudity According To MPAA Rating

MPAA Rating Bumper Rated X Remake (1970-1971) by TheAnthonyCorp on DeviantArt

A History of the X and NC-17 Ratings – The Hollywood Reporter

CLOCKWORK ORANGE' TO GET AN 'R' RATING - The New York Times

Rated X | Sticker

Project X Parents Guide

IPX Rating System: What It Means And Why You Should Know

The first American film to receive an X rating

All the X-Men Movies Ranked - IGN

Where to watch X and the horror movie's age rating

What is the difference between the movie ratings R (17+), X (19+) and XXX (21+) other than age, and what do they mean? - Quora