I don't think there is a particular "time frame" as such. Yes, the document had a date on it, but so did newspapers in Storybrooke, and people just didn't really notice if time was being wibbly-wobbly. Sort of a "somebody else's problem" haze over the whole date and time thing.
As to why it would exist separately from the LWM, I've been thinking about it a lot.
An enormous amount of our most well-remembered classical fiction in the LWM is based in and around Victorian England. The Industrial Revolution wasn't just about industry and technology; culture leapt forward as well. It generated a huge number of great, romantic, and inspiring tales in a relatively short time frame. It's a setting of fantastic good, where hope, invention, and imagination created new technological and industrial wonders seemingly every day. It's also a setting of fantastic evil, where an oppressive class system and brutal imperialism killed hundreds of thousands in the name of progress. Victorian England is a setting that occupies a huge cultural and narrative space, and it makes perfect sense that the most narrative bits of it would splinter off as a World in its own right. (I do hypothesize that that World includes not just Victorian England, but the rest of the British Empire of the time, as the imperialist nature of the setting is inextricable from the rest of it.)
Basically anything by Charles Dickens, including A Christmas Carol, A Tale of Two Cities (technically France, but similar time frame), Oliver Twist, etc., could show up. You'd also have the Bronte sisters, Jane Austen (technically Regency, but again, close enough), William Thackeray, Thomas Hardy, George Eliot, Robert Louis Stevenson, Rudyard Kipling, Arthur Conan Doyle, and others. Depending on just how big Victorian Times World is, you might also have American authors and stories set in the same time frame, like Mark Twain. In a show like OUAT, characters like Ebenezer Scrooge, Sherlock Holmes, Huckleberry Finn, or Mowgli could all show up one day, and could all potentially come from this one Victorian Times World. It just depends on the demands of the show's story.
(Watch them eventually do a Christmas episode with Scrooge. It will inevitably become too great a temptation for any modern writer with access to a version of Victorian England.)