After looking through a bunch of sources, I have found the following information that might help me date the manuscript I'm working with:

1. The script is almost certainly a Protogothic script, typical of the 12th and early 13th centuries. Analysis of the long flourishes on the "feet" of the letters strongly suggests that this is a Southeast English hand (e.g. Canterbury or Rochester) and tends toward an earlier date or else a scribe writing in a more conservative hand.

2. I will need to re-examine the text to see how often (if at all) contrasting pen flourishes on the colored capitals occur. Pen flourishes like these became common in the Gothic period from the 13th century onward, so their presence or absence might be helpful in establishing a date. I do know that many of the capitals (such as one particular "A" I have photographed) do not display this feature.

3. I will need to re-examine the text and my initial notes to discover if pricking exists on one or both sides of the margin. If there is pricking on only one side, then the text was almost certainly from before 1150 (s. XII 1) as pricking on both sides was common in both England and the Continent from 1150 to 1250. This might be my smoking gun as to whether it's a 12th or 13th century manuscript.

It looks like Saturday, I'll be taking another trip to the library to examine the book again and probably take more photographs to cross-reference with the source books I'm using. Overwhelmingly, the evidence points more toward a 12th century date and may push the date back to well within Peter Lombard's lifetime. It seems I'm well on track for meeting my goal of nailing down a date by this weekend.