The problem concerns the period before 100 CE. Let us leave aside the later period and biblical scholars’ doubts about the authenticity of early patristics. How the original Pauline Corpus was formed. Zuntz insists on the creation of one, only version of the anthology of Paul’s letters. He places special emphasis on this. One Pauline Corpus.
A common version that Marcion and Clement and Ignatius and everyone else was supposed to use. Where did they buy this shared version?
The only process that makes sense is the variant I drew – there was a letter distribution network where copyists produced copies, copies were delivered to recipients, recipients inserted subsequent letters into the codex.
Only in this way could copies of the same Pauline Corpus reach different people.
This means that the letters entered the market after editing, and that the letters of other authors were brought into circulation as part of this process.
This explanation contains less artificial assumptions than some lucky seeker, an editor who searches for letters over a vast area and still succeeds.

Unless the anonymous author of the prologue to the Gospel of St. John:
“The Gospel of John was revealed and given to the churches by John while still in the body, just as Papias of Hieropolis, the close disciple of John, related in the exoterics, that is, in the last five books. Indeed he wrote down the gospel, while John was dictating carefully. But the heretic Marcion, after being condemned by him because he was teaching 2 the opposite to him [John], was expelled by John. But he [Marcion] had brought writings or letters to him [ John] from the brothers who were in Pontus.”
Why write the last sentence about the hateful enemy and show his contribution to the delivery of correspondence from the brethren in Pontus?
It’s the same PR error as Tertullian. Let’s write some good-sounding explanation of the compromising facts – here we refunded Marcion’s money and the letters and writings we kept were not his. Only brethren in Pontus…