It seems like the blanks add to the “reality effect” of the fictional work…a trick to make the reader believe that the story’s events are real. If the reader sees “Mr. B.” instead of Mr. Brown, that suggests that there really is a Mr. B. somewhere out there in the real world, but the author has chosen to render him anonymous.
Also, there’s the tradition in letterwriting of using initials rather than full names (just in case the letters fall into a third party’s hands). Since a number of the earliest novels were “epistolary novels” (novels composed of letters supposedly exchanged between fictional characters, as in Richardson’s Pamela ), maybe the earliest novelists just carried over this practice.
(This is just a theory…maybe someone else here knows for sure…)
My old-book tastes run mainly to old kids’ series books. I always assumed that the 18-- date convention was to keep the books from dating too quickly, so that they could be reprinted for years and the readers would still imagine that they were reading a current book.
The thing with using only the initial letter of the last name – my guess would be that’s it’s to avoid causing offense (or stimulating libel suits) with people who hold the same position as the fictional character. So, for instance, if the governor of Vermont features in your book as a criminally stupid egotist, by referring to him as “The Honorable Mr. P-----” or something like that, people won’t confuse him with the real person (or, if a real governor is a criminally stupid egotist named “Perkins,” the author can always claim that that’s not the reference readers were intended to draw.)
There’s an odd trend in older fiction that seems related: the author describes something as uncertain when there’s no reason it would be. For instance, he’ll say the library was richly appointed with book shelves and an overstuffed reading chair, and perhaps a table or two. What do you mean, “perhaps a table or two”? You’re making this up, so you choose exactly what’s there, whether it’s two tables or a sick wolverine. What’s with the coy faux uncertainty?
I always figured it was the literary analogue of the way Bill Cosby would run his hand in front of his mouth to avoid saying something specific, both in his stage act and on the Cosby Show. Made-up example:
Cliff, what time did you get in last night?
Oh… it was about mphmpmhpmh<hand across mouth>:30