The Spectre was another Earth-Two denizen who made a bunch of unexplained appearances on what appears to be Earth-One, beginning in THE BRAVE AND THE BOLD #75, where he met up with Batman to face off against the menace of Shahn-Zi. This Spectre was split from Jim Corrigan, much like the Spectre was in his own magazine at the time, but no explanation was ever given for his sudden appearance on Earth-One (in both his heroic and secret identities).
The Spectre's appearances anywhere could've easily been explained away as being the Earth-Two hero moving between the alternate universes (given his usually great power and ability to do so), but this was never mentioned or stated in any of these particular tales.
A much grimmer Spectre began avenging crime in the early seventies, as Michael Fleischer and Jim Aparo related the character's adventures in ADVENTURE COMICS. This Spectre was much more similar to the earliest version of the character, and the body count was both high and warped. Jim Corrigan was still the alter-ego of this Spectre, and he did show up to help the Batman on one occasion which would seem to indicate that he belonged on Earth-One.
The one problem with this particular portrayal of the Ghostly Guardian being set on Earth-One was that in one story, it was shown that pretty much everyone knew that Clark Kent was Superman, indicating that he was probably a fictional character there.
Several years later, Dr. Terrence Thirteen was involved with the Spectre while attempting to debunk a number of charlatans. This Spectre showed the same rather-murderous attitude toward evil that the Fleischer/Aparo Spectre had, which is why I group these appearances in with those of a probable Earth-One Spectre.
One key to the mystery may lie in an almost throwaway caption in AMERICA VS. THE JUSTICE SOCIETY #2. After the Spectre offers to basically destroy the world for his old friends to get them out of legal trouble, the Spectre leaves, to the caption "Then, it seems as if a different universe opens up beneath the feet of one who has been Jim Corrigan on at least two Earths — and the Spectre is seen no more among men." To me, this says that while there is only one Spectre, there may be a multitude of Jim Corrigans that he can inhabit at any time.
The Spectre had "died" during a JLA/JSA crossover circa JLofA #83. His next appearance was in the Fleisher/Aparo ADVENTURE COMICS series, with no other super-heroes in evidence. However, Joe Orlando (or someone) confirmed in one of the letter columns that "this is the Earth-One Spectre". This was probably just to satisfy the fans' questions, since there is no real indication that Fleisher cared about multiple Earths. The Clark Kent/Superman comment was, I suspect, partly a gag and partly an attempt to preserve the grim and creepy atmosphere of the series, by fostering the illusion that these stories took place in the "real" world... with no "soft-hearted", gaudy super-heroes to clutter things up.
That said, the E-2 Jim Corrigan turned up alive(?) and well circa #70 of the 1970s ALL-STAR COMICS series, with the Justice Society, mentioning that the Spectre seemed to have disappeared lately... the implication being that he now inhabited the E-1 Corrigan. And in JLofA #220 a few years later, the specifically Earth-One Corrigan turned up again, being held in some kind of mystical stasis. Roy Thomas promised in a footnote to explain how he got that way in a future story, but it never happened.
Steve Gerber pitched a new SPECTRE series to DC in the 80s that would have, among other things, explained the two Corrigans, but after CRISIS, the whole question was declared moot, since there was now only one Earth and one Corrigan anyway.
(And there was also ANOTHER police detective named Jim Corrigan on Earth-One in the 70s, who apparently had nothing to do with the Spectre. He appeared in the later JIMMY OLSEN issues and in BLACK LIGHTNING, working in Metropolis. Actually, I'm not sure if they were both the same guy, but they were almost certainly intended to be.)
When you think about it, the Crisis did more good for "Earth-B" than anything else, since many of those questionable tales (especially with Wildcat and Plastic Man) now can fit into the DC universe fairly easily.
Of course, Batman still couldn't have met the Blackhawks, the Unknown Soldier, or Sgt. Rock during the war, and the Vigilante's post-1947 adventures are still up in the air, but what the hey?
THE BRAVE AND THE BOLD, during the years of Haney and Boltinoff, could be confusing. Most times, they ignored the whole Earth-One/Two device but other times, (the Black Canary story), they would use it themselves.
- The Brave and the Bold #75, 116
- Adventure Comics #431-438