While the beginnings and endpoints of various vintage comic book eras can often be a matter of debate and differing opinions among collectors, the ending of the Pre-Code era is very simple. By definition, the beginning of the Comics Code Authority marks the end of the Pre-Code era. The Comics Magazine Association of America adopted its Code on October 26, 1954, after public campaigns by Fredric Wertham and others against horror, crime, and risque comic books. This moral panic led to televised hearings during which Wertham and comic book industry figures testified before the U.S. Senate. Conforming with Comics Magazine Association of America Code guidelines became a de facto requirement to get distribution to newsdealers in the United States, and comics that received pre-publication approval from Code administrators carried the Comics Code Authority seal on their covers. American horror comic books up to that moment can be PCH, but of course, comics that bear the Code seal cannot be Pre-Code. So for example, Uncanny Tales #28 is Pre-Code Horror, but Uncanny Tales #29 (the first issue of the series to bear the Code seal) is not.
What difference did having the Comics Code Authority approval make? Code approval required strict adherence to a number of guidelines that covered violent, sexual, and lurid content. In the area of horror, such guidelines included:
Put simply, while there were still horror comics of a sort published after the advent of the Code, horror titles which lasted from the Pre-Code to the Post-Code era shifted focus from pure, hard-edged horror to fantasy (ACG's Forbidden Worlds, for example) or weird science fiction (Marvel titles such as Journey into Mystery among others). Pre-Code Horror typically has a distinct look and feel vs the comic books that came after it. To put that in perspective, even when the Comics Code Authority guidelines against some aspects of horror were relaxed in 1971, allowing staple horror characters to appear again (Marvel's Morbius the Living Vampire, appearing in Amazing Spider-Man #101, and Werewolf by Night, appearing in Marvel Spotlight #2 are good examples) the overall look and feel of comic book horror still did not approach the extreme edge that it had at the height of the PCH era. And while there are certainly horror comics published in more recent times that have recaptured that type of horror, it's arguable that mainstream comic book horror has never been the same since the PCH period. It was a historically unique time in comic book history.
While the end of the PCH era is easy to define, the beginning of the period is somewhat more elusive. Key early moments in comic book horror history look something like this:
Given the above, it's somewhat difficult to establish a starting point for the PCH period, and this points to how this area of collecting is likely to evolve in the future. While Eerie Comics #1 is widely considered the first horror comic book, it's difficult to exclude Harvey's Front Page #1 from that category — particularly considering the contents of comics that are firmly considered horror such as Adventures into the Unknown. Likewise, Amazing Mysteries #32 is likely much more important than it gets credit for as the start of the boom period of PCH, as Marvel became the driver of the genre with around 389 comics which can be considered PCH, vs EC Comics PCH output of about 91 comic book issues.
Overall, there are over 1400 Pre-Code Horror comic books from over 30 publishers during the PCH period.
The Digital Comic Museum has assembled a good list on the subject, which is itself based on a list from the book Four Color Fear: Forgotten Horror Comics of the 1950s. Even with this relatively comprehensive list, there's some room for expansion. The Chesler line includes a number of covers likely to appeal to PCH collectors, like the classic Punch Comics #19. A number of comics featuring covers by the likes of L.B. Cole, Alex Schomburg, and others such as the legendary Suspense Comics #3 are also examples of comics that are not technically PCH, but are likely to appeal to PCH collectors on the strength of the cover alone.