Firey advances four arguments to convince us that Hamilton 132 is the product of collaboration. First, she finds problems with the traditional explanation that Caroline scribes in Hamilton 132 are reworking a Dionysio-Hadriana to look more like a Hispana:
The Hispana has a set of Gallican councils not in the Dionysiana, and also a substantial sequence of about fifteen Spanish councils. … If the Caroline scriptorium were updating or expanding a Dionysiana to make it more like an Hispana, it would be the Caroline scriptorium that added the Gallican and Spanish councils to the manuscript. The Gallican councils, however, are in “a-b” script, and the Spanish councils are in both Caroline and “a-b” script. What the Caroline scriptorium added were the sorts of supplementary texts…that often get appended to canon law codices to enhance their intertextual value (45-46).
Three pages prior to these remarks, Firey acknowledges that the Collectio Sancti Amandi was a collection of Gallican and Spanish councils assembled to supplement the Dionysio-Hadriana, available in at least two manuscripts besides Hamilton 132. When Gallican councils in a-b script or Spanish councils in a-b script occur in Hamilton 132, it turns out that they are always from the Collectio Sancti Amandi. This observation is the basis of the standard characterization of Hamilton 132 as a Collectio Dionysio-Hadriana cum Collectione Sancti Amandi that has been expanded with Hispana material. Again, just to be totally clear: Because the Collectio Sancti Amandi contains Gallican and Hispanic councils, a-b scribes sometimes copy Gallican or Hispanic conciliar texts, which they have from the Collectio Sancti Amandi. The Caroline scribes do add “supplementary texts,” but they also add a substantial amount of canonical material from the interpolated Hispana.
Firey has a second point in favor of collaboration:
The evidence that seems to controvert the possibility that the Caroline portions were inserted in an independent, later process is in the pricking and ruling of the codex. The pricking and ruling seem to reflect the existence of two, separate scriptoria. The ‘a-b’ leaves are ruled in a particular way: four pricks and lines for the center boundaries of the columns; the Caroline leaves, although ruled for the same text space and also for the same forty-two lines per column, show a much simpler form in the column boundaries (single rather than double lines). The matching text spaces and number of lines do, however, suggest co-ordination. Again, the Caroline leaves could have been planned later specifically to match the codex that was to be updated. At several points, however, the Caroline scripts are on leaves that are pricked and ruled in the ‘a-b’ style. That suggests that some parchment was passed from the ‘a-b’ scriptorium at the time of the ‘a-b’ transcription to the Caroline scriptorium for use in the same codex…. (46-7)
In a footnote that sometimes confuses which leaves in Hamilton 132 have a-b and which have Caroline text, Firey explains that the “a-b” ruling is evident on Caroline folios 31, 33 through 49, 40 through 51, and 56 through 61. After considering explanations for this phenomenon that do not involve “collaboration,” Firey concludes:
Simultaneous production in two different, co-operating scriptoria seems more likely, and fits with Bishop’s analysis of other manuscripts showing similar combinations of “a-b” and Caroline script.
As for Bishop, we have seen that he explicitly rejected collaboration between the a-b and Caroline scriptoria at Corbie. But however that may be, there is nothing inherently “a-b” about double bounding lines, beyond their association with a-b scribes in this specific codex. With that point in mind, the worst we can do to Firey’s argument is put it in simpler terms: According to Firey, the a-b scribes always write on folios with double bounding lines. Also according to Firey, the Caroline scribes generally write on more simply ruled parchment, but sometimes their parchment is also ruled with double bounding lines. You may decide for yourself whether and to what degree traditional views of Hamilton 132 are refuted by this observation.
Firey has a third argument:
Finally, as evidence for apparently simultaneous production, fol. 44r has two corrections, which appear to be made at the same moment, to judge by the system of symbols for insertion of the correction, the size and pen-angle of the scripts, and the ink. One, however, is in Caroline minuscule, and one is in ‘a-b.’ Either they were made by a person who could switch script styles readily, or by two persons who may have passed the same quill back and fort at a final phase of proof-reading’ (48).
From Firey’s own reproduction of fol. 44r (fig. 1 at page 76), we can see that a Caroline scribe has supplied one omission in the upper margin, while an a-b scribe has supplied another in the lower margin. The tie signs are different, the module is different, and the pen-angle is different. As for the ink, the relative colors are hard to judge in black-and-white. Even if all these things were the same, though, would we be compelled to imagine script switching or quill passing? What has become of Firey’s skepticism in the face of other scholars’ argumentation? Here, it seems that the only thing we can say with certainty is that the Caroline corrector and the a-b corrector worked after the main text on fol. 44r had been written.
Firey has an ultimate point:
Furthermore, in several instances, the Caroline hands transcribe material from the Dionysiana that is not in the HGA as represented in Vat. lat. 1341: at some point, the ‘a-b’ exemplar would seem to have been passed to the Caroline scriptorium for final revisions to Hamilton 132.
This is another observation that does not compel anybody to posit collaboration in the production of Hamilton 132. Otherwise, the phenomenon Firey notices has an explanation, but it does not lie in the putative circulation of exemplars between scriptoria. We will look at one instance in which Caroline scribes copy Dionysio-Hadriana text, highlighted by Firey in support of this final point, to explain how it came to be that Caroline scribes, adding Hispana material to a Dionysio-Hadriana manuscript, found themselves in the position, occasionally, of recopying Dionysio-Hadriana material. Here I am dependent upon Paul Hinschius’s study.
At fol. 106, Caroline scribes copy J3 1182, a decretal of Simplicius from the Dionysio-Hadriana that is not present in the Hispana. Why do they do so? We must begin by recognizing, along with Firey, that Hamilton 132 is badly misbound. The folios that ought to precede 106 are therefore not the folios that actually precede it in the present binding; rather, the immediate sequence begins at fol. 96 and continues through fols. 97, 104, 105 and then finally 106. All but fol. 106 are a-b folios. On fol. 96 we have the beginning of the Hilarius decretals from the Dionysio-Hadriana (though these letters recur in the Hispana as well), and this sequence continues through fols. 97, 104 and 105. Yet the Hispana has more Hilarius letters than the Dionysio-Hadriana, and so at fol. 105 our Caroline redactors erase the beginning of the Simplicius dossier that originally followed Hilarius in their Dionysio-Hadriana, in order to create more room for Hispana supplements. These supplements have to go here if the chronological order is to be maintained and the Hilarius material is to be kept together, all in one block. The Caroline scribes then continue to add two pieces of Simplicius correspondence from the Hispana, directly after their Hispana supplements to the Hilarius decretals. Then, finally, they recopy the Simplicius decretal from the Dionysio-Hadriana, J3 1182, that they obliterated to make way for their expansions to Hilarius. They do this on fol. 106, which is tipped in.
The process of supplementing chronologically arranged canonical collections, in other words, occasionally required both cutting and pasting to maintain the pope-by-pope sequence of decretals. This implies nothing about collaboration, though in this case it does show that the Caroline scribes wished to retain Dionysio-Hadriana material alongside their supplementary Hispana texts.
 See Kéry, Canonical Collections of the Early Middle Ages (Washington DC, 1999), 84.
 Die kanonistischen Handschriften der Hamilton’schen Sammlung im Kupferstich-Kabinett des königlichen Museums zu Berlin, Zeitschrift für Kirchengeschichte 6 (1884), 193-246, at 228.