In the original edition and subsequent, until 1961, the S of the first chapter is a bit higher as well as the M in the 2nd. and the P in the 3rd.chapter, all full-page and a dot at the end of episode 17.

They have the function of indicating:

S = Subject = Stephen Dedallus
M = Middle = Molly Bloom
P = Predicate = Poldy's = Lopold Bbloom

Dot = . = Q E D = quod erat demonstandum, which means "which had to be demonstrated" or "it has been proved".

It was a medieval pedagogyl S-M-P indicating the order of cognitive thought that should be learned.

It suggested a logical narrative structure that the reader could see, but the fictional characters completely ignored.

S + M = P or, the "X" of the question is M, Nora Barnacle or Molly...

More information on Ulysses Annotated by Don Gifford, pg 12, under S.