Right, so this is interesting.
After the Isidore/Merchant preface we get a fictive exchange of letters between "Archbishop" Aurelius of Carthage (who assumed office in the early 390s) and Pope Damasus (d. 384). You see the problem here.
The first letter is quite short. In it Aurelius asks for papal letters from the time of St. Peter's successor (Clement I) up through Damasus's own pontificate. He wants these letters "so that we might know what was decreed...in times past, and so that those who act contrary to the institutes of the sacred canons might be recognized." Pseudo-Isidore misses no opportunity to sharpen his knives, that's for sure.
In the rescript we have Damasus claiming to be sending all of the letters issued from the end of Peter's pontificate through Damasus's own time, and also (predictably) complaining about the "violators of the canons" who are "gravely judged by the holy fathers and by the Holy Spirit." And that's it.
The significance of this? Well, remember that Part 1 of Pseudo-Isidore (forged decretals from Clement I through Melchiades) is the most blatantly fictional bit of the entire collection. Part 2 (councils) is interpolated but firmly grounded in genuine material; Part 3, though it contains a higher concentration of the fictional, also has hefty doses of the perfectly genuine. This little epistolary exchange marks off the clearest forgeries and claims that they're a separate canonical collection assembled by Pope Damasus. Within the decretals of Pseudo-Isidore, in other words, we have the decretals of Pseudo-Damasus.
Note that Pseudo-Damasus doesn't fit neatly within Part 1 of Pseudo-Isidore, which has decretals only through Melchiades. To get decretals through Damasus we have to turn to the beginning of Part 3. So it looks like Pseudo-Isidore has split up his Pseudo-Damasus, at least in the A/B and A1 recensions.
Now look at the A2 recension: It has no Part 2 and lacks all decretals after Damasus (in fact it only has the first two Damasus letters). A2, in other words, is the Pseudo-Damasus -- it's exactly the collection described by this letter exchange. The only problem is that A2 also carries the general preface that we described last time -- the preface that promises councils in Part 2 (which A2 lacks) and decretals through the time of St. Gregory in Part 3 (most of which A2 lacks).
I'll leave you to ponder that.
Recipients: Aurelius, Damasus
Dates: Aurelius letter, given on 22 February and "read at Rome" on 3 May (no year); Damasus letter given on 17 May, during the consulates of Gratianus and Siricius (invented consuls, and hence no year once again).
Two months, in other words, for Aurelius's fictional letter to make it from Carthage to Damasus's desk at Rome. Does that seem a bit long to you?
Sources: None for the very short Aurelius letter; for Damasus, Leo the Great, Ep. 44; also a letter of Pope Felix III to the bishops of Sicily (from the Hispana, also in Part III of Pseudo-Isidore)
Lines (in Schon's edition): 10 (for Aurelius's request); 27 (Damasus's response)
Cross References: To forged decretals from Clement to Damasus