In 2013 Hans-Gebhard Bethge published an article in Early Christianity entitled "Ein neu bekannt gewordener Papyrus-Codex mit Texten aus Paulus-Briefen. Ein Werkstattbericht." An English version of this article is also available. Brent Nongbri blogged about this manuscript in 2019. In the SMR database in Münster, the manuscript has received the number sa 491.

Along with other information, Bethge mentions the dimensions of the pages (24.5x15.5 cm), the average amount of lines (30-35) to a page, and the average number of letters per line (25). In addition, the page numbers for the final two pages of Hebrews can be read as 139 (ⲣⲗⲑ) and 140 (ⲣⲙ), following which, the numbers 141 (ⲣⲙⲁ), 142 (ⲣⲙⲃ), and 144 (ⲣⲙⲇ) are visible on the first pages of Galatians. Hence, Gal must have followed Heb, and Heb 13 was preceded by text on about 138 pages!  Bethge is worth quoting in full (p. 6):  

"If Heb is somewhere around page 130 and – as one may assume – 1 and 2 Cor as well as Rom, or at least Rom, had preceded them, then space would remain for something additional. It is unclear, though, what texts or writings these could have been, and it can therefore only be speculated upon. Surprisingly enough, one page in this codex, at a rough estimate, is nearly equivalent to one page in Nestle-Aland’s Novum Testamentum Graece. Taking into account the aforementioned long epistles, the pages occurring at the beginning may therefore have been preceded by approximately 100 pages of Pauline texts. It still remains a mystery as to what else could have been there. Earlier on, Gregor Wurst and I had considered a theory that had supposed the Acts to be the codex’s opening script. However, this can hardly be the case as it is far too long. Careful thought could now be given to an idea that the so called Catholic epistles may have been positioned right at the beginning. The space available would probably have been sufficient. Given the state of current knowledge, however, this exciting question still awaits an answer."

This is a perfect challenge for our Manuscript Speculation Tool. This tool allows us to enter manuscript parameters into an input form for the computer to create a manuscript based on these parameters and the text that we think it must have contained. I have started with Bethge's suggestion that 1 and 2 Cor and Rom could have preceded Heb in a manuscript that has 25 letters per line and 32.5 lines to the page. (It is a nice thing that we can enter decimals; in this case, 32.5 is the midpoint between 30 and 35). In addition, I have added three extra lines for a header (superscription) above Rom and two lines in front of every new writing to allow for subscription and/or superscriptions. The string of books and "headers" that will do all that is LB1;Rom;LB2;1Cor;LB2;2Cor;LB2;Heb;LB2;Gal. Finally, I told the computer to start with page 1 (α) and create 140 pages. This is the result: Heb ends on page 137!

Allowing for the possibility that the first leaf (pages 1-2*) could have been part of the manuscript cover and was thus not numbered, this is a perfect match! In fact, the match is so good that I have not even looked for an alternative scenario. As a result, we can safely put aside any speculation as to what other books let alone from outside of the Corpus Paulinum might have been included before Heb. Instead, this very old papyrus codex (4th c.?) seems to have followed the "normal" arrangement of the Pauline Epistles in the Coptic (Sahidic) tradition: Rom 1-2 Cor Heb Gal sq.