In the Time Patrol timeline, Sherlock Holmes existed in nineteenth century Britain.
In the Technic History timeline, Apollo University in Leyburg on the Moon will exist - in 2057.
None of these is our timeline, of course, but the Technic History will be conclusively proved to be fictional/alternative forty one years from now!
Usually fictional characters are fictions to each other, e.g., James Bond likes Rex Stout's books about Nero Wolfe. Various fictional detectives refer to Sherlock Holmes - as a fictional detective - and Holmes himself refers disparagingly to Poe's Dupin. When Anderson instead includes Holmes as a real person in the Time Patrol series, is this an instance of "metafiction," i.e., a fictional text acknowledging its own fictional status? It might be like a picture frame or the proscenium arch, defining the boundary between the fictional world and the space inhabited by its audience? Pretty smart stuff.