http://www.archive.org/post/341508/100-movies-in-the-public-domain

“The works specified by sections 102 and 103, when published, are subject to protection under this title if—

(1) on the date of first publication, one or more of the authors is a national or domiciliary of the United States, or is a national, domiciliary, or sovereign authority of a treaty party, or is a stateless person, wherever that person may be domiciled; or
(2) the work is first published in the United States or in a foreign nation that, on the date of first publication, is a treaty party; or
(3) the work is a sound recording that was first fixed in a treaty party; or
(4) the work is a pictorial, graphic, or sculptural work that is incorporated in a building or other structure, or an architectural work that is embodied in a building and the building or structure is located in the United States or a treaty party; or
(5) the work is first published by the United Nations or any of its specialized agencies, or by the Organization of American States; or
(6) the work comes within the scope of a Presidential proclamation. Whenever the President finds that a particular foreign nation extends, to works by authors who are nationals or domiciliaries of the United States or to works that are first published in the United States, copyright protection on substantially the same basis as that on which the foreign nation extends protection to works of its own nationals and domiciliaries and works first published in that nation, the President may by proclamation extend protection under this title to works of which one or more of the authors is, on the date of first publication, a national, domiciliary, or sovereign authority of that nation, or which was first published in that nation. The President may revise, suspend, or revoke any such proclamation or impose any conditions or limitations on protection under a proclamation.

For purposes of paragraph (2), a work that is published in the United States or a treaty party within 30 days after publication in a foreign nation that is not a treaty party shall be considered to be first published in the United States or such treaty party, as the case may be.

M seems to qualify for (1), (2), and (6)

M was released in the USA more than 30 days after it was in Germany so it qualifies as a work originally published outside the USA. As, at the time of publication, the US had a bilateral copyright treaty in force with Germany, which began in December 1910, the authors’ and work’s country of origin was a “treaty party” and so copyright was extended to the work.

It was registered for copyright properly:

M. Nero Films, AG., Berlin. 1931.
9 reels sd Authors: Director, Fritz Lang;
scenario, Thea Von Harbou
Copyright Foremco Pictures Corp. and Nassau
Films, inc (Nero Films, Author); 11May31;
LP3800

When renewal came up it was not renewed and fell into the public domain in the USA. However, after the URAA was passed, the successors of the copyright registered a NIE on the film:

M. Additional title: Murderer among us, Morder Unter Uns, A town is looking…
Type of Work: Recorded Document
Document Number: V8007P558
Date of Recordation: 1997-12-31
Entire Copyright Document: V8007P558 (Single page document)
Title: M. Additional title: Murderer among us, Morder Unter Uns, A town is looking for a murderer.
Notes: Motion picture. Filed for all rights.
Notice of intent to enforce a copyright restored under the Uruguay Round Agreements Act.
Owner: Atlantic-Film, SA. Via Resore 6; Postfach 137; 6949 Comano, Switzerland. PHONE: (41) 1 422-3832. FAX: (41) 1 422-3793.
Author: Fritz Lang.
Date of Publication: 1998-04-17

Variant title: M.
Murderer among us
Morder Unter Uns
A town is looking for a murderer.
Names: Lang, Fritz
Atlantic-Film, SA.

Owner: Atlantic-Film, SA. Via Resore 6; Postfach 137; 6949 Comano, Switzerland

17 U.S.C. § 104A (b) Ownership of Restored Copyright. — A restored work vests initially in the author or initial rightholder of the work as determined by the law of the source country of the work.”

Dementia 13: http://www.youtube.com/watch?v=c0ukfVt3d5Q

Phantom Of The Opera: http://www.youtube.com/watch?v=ie3TfYfuqzA

The Last Man on Earth: http://www.youtube.com/watch?v=dX6nsSCapiA

House on Haunted Hill: http://www.youtube.com/watch?v=7BeUWMfqELk

Dr Jeckyll and Mr Hyde: http://www.youtube.com/watch?v=5OVOn15Yet4

Night of the Living Dead: http://www.youtube.com/watch?v=85C2JnZOY4k

M: http://www.youtube.com/watch?v=jewTfVC7XD4