In my entry for Ab.57.1 Memoirs of an Unfortunate Young Gentleman I listed three variants: my frankenbook contains a fourth. A rather interesting one. It appears that, during printing, N9r and N10v were mis-imposed in the outer forme, transposing pages  and  of the advertisements. Each of these pages start with a “the” (: “The Plain Truth …”; : “the Author of …”), which means each of the catchwords are both "the" (actually, “The” and “the”)—so it is easy to see how this error might have occurred. Especially since N9r and N10v are side-by-side in the outer forme.
What is puzzling, is why, when the error in imposition was corrected, that the ornament on N10v was changed from a pair of heart-shaped ornaments to a pair of acorn-shaped ornaments.** At first I thought this might be evidence of printing from more than one setting of type, possibly on more than one press, but N10v (and the other pages in the gathering) are exactly the same setting of type in both states.
This can be confirmed by looking at the images above and below. (Note that the damaged “o” in “Dialogue” in the forth line, and the damaged “4” in “1742.”) Another possibility is that the advertisements were printed from standing type, but the fact that these pages were mis-imposed and then corrected suggests otherwise.
Whatever the explanation, it is a good demonstration that a badly damaged copy—as well as being a witness to its own unique history (as a schoolboy’s companion, for example)—is also an important witness to the edition of which it is a part.
** I cannot find a comprehensive list online of names for Caslon type ornaments and terminals—such as arabesque, rosette, acorn, etc—to properly describe the ornaments above, but I trust the pictures make up for the lack of an accurate terminology.
[UPDATE: 2 July 2016: After my pictures disappeared again, I decided to give up on external hosts for large versions (1000px) of my images, and stick with the smaller ones (500px) that Blogger is prepared to host.]