I have had a scintillating time perusing the UPenn papyri holdings at their nicely designed online collection hosted by the Schoenberg Center for Electronic Text and Image. These are very sharp digital images, and allow a set zoom to the level of two lines of the text square.
In one good example of how the clear these digital images are, here is the P.Oxy. manuscript of Matthew 1 (P.Oxy. 2?). As Robert Kraft just pointed out to me, you can clearly see what appears to be an iota (or upsilon) with a superlinear mark on the upper left margin, as well as the alpha (page number) on the upper center. He suggested that this may be the sort of feature I am looking for as a manufactural mark (for lack of a better term) of its actual construction as a codex.
There are a few interesting features posed by these markers. From what I can tell from the online photograph, the "A" page number could be from a different hand from the rest of the text. The top stroke of the right half of the alpha ends a bit low and round compared to the other alphas on the page. Likewise, the hand of this manuscript produces a rather crisp, confident iota. If the secondary marker in the top margin is an iota, it also doesn't match the hand of this manuscript. From what I can tell, the same is true even if it is an upsilon. If it is a slightly faded lowercase gamma, however, it would be hard to tell from which of these two hands it comes.
All this is to say that such markers as are found in this margin, if truly coming from a different hand, could be assessed as marks of manufacture. I am a bit at a loss as to its actual reference. An iota perhaps marking this as the first quire of ten sheets (40 pages)? This would be a bit bulky, but possible. A gamma marking 3 sheet quires (12 sheets)? This comes closer to the classic octavo pattern of book-binding, much more amenable to early binding materials than larger quires.
Disclaimer: I am simply musing here with no firsthand knowledge of this actual manuscript, and a basic grasp of these features. I am interested in actually finding some marks of manufacture, and it is interesting to muse about the features described above from this perspective. Any corrections to the above is greatly appreciated, if not requested. If anything, take this post just as a link to the Penn project.