Like that of the Venetus A, the front matter of the Iliad manuscript known as E4 (Allen, = West F, Escorialensis Ω.I.12) has been a source of confusion and speculation for scholars. (For the Venetus A, see Allen 1899 and Hecquet 2009 in Recapturing a Homeric Legacy). While trying to understand this odd assembly of Homeric reference material—material which includes lives of Homer, a Proclan summary of the Cypria, an excerpt from the Batrachomyomachia (“Battle of Frogs and Mice”), and an excerpt from a work of the grammarian Tryphon, among other things—I came across a text on folio 2r entitled ἐξήγησις τῆς ἰλιάδος καὶ ὀδυσσείας ὁμήρου or "Exegesis of the Iliad and Odyssey of Homer." Not knowing what this work was, I typed the whole phrase into Google, and discovered this manuscript (Harley 5601), now on-line thanks to the British Library's Digitized Manuscripts initiative (funded by the Stavros Niarchos Foundation).
Like E4, Harley 5601 contains hypotheses before each book of the Iliad. It is clear, however, that those of Harley 5601 are not copied from E4, because the hypothesis for book 2 in Harley 5601 is much more fuller than the corresponding hypothesis in E4 (which seems to have been abbreviated by the scribe because of lack of space). (In E4, all books of the Iliad begin on the recto side of a folio. On the adjacent verso side, the scribe writes usually two hypotheses along with scholia if there is room. In this case, the end of book 1 three quarters of the way down the page left him very little space for the hypotheses to book 2.) The actual wording of the hypotheses (that of Harley 5601 and the second hypothesis for book 2 in E4) is the same until the scribe runs out of room on E4.
Harley 5601 postdates E4 by many centuries, but it seems clear that share a partial ancestor that predates E4. That ancestor had the prolegomena that these manuscripts share, and, very likely, one set of hypotheses. Their texts of the Iliad, however, have different ancestors. Perhaps E4's had the additional set of hypotheses that Harley 5601's did not, or perhaps that other set of hypotheses in E4 came from still another source.
Many questions remain about how these two manuscripts might be related. I am grateful to the Niarchos Foundation and the British Library for making these images freely available, so that further study can now take place—by anyone that wants to. These two manuscripts have almost certainly never been, and never will be, in the same room together. But now, if you have a big screen, you can zoom up on them in great detail, side by side.