The copyright for a published work could be registered anytime in its first 28 years of publication, so the owner(s) of the copyright to Plan 9 From Outer Space could register it anytime in its first term of copyright. Plan 9 was registered in 1981. However, that copyright term ended in 1986 (1958 + 28).
The person who renewed the copyright in 1986 was film distributor Wade Williams, who specializes in cult and sci-fi movies. However, as far as I and many other people know, Williams had no legal claim to do so. The U.S. Copyright Office does not normally investigate these matters; it files the paperwork submitted. If Williams would ever file a lawsuit against someone for copyright violation of Plan 9, the defendant could demand proof that Williams was the legal heir to the copyright.
I understand that Willaims got Ed Wood’s widow to sign his papers. Two problems with this: Ed Wood’s wife never owned the copyright either; and she and Ed Wood were never legally married.