Epistolae Karolini aevi. IV / hrsg von E.L. Dümmler, Ernst Perels. (OCLC #810904180)
This series is so large and complicated (and being purchased slowly over time and out of order) that I keep a copy of its index at my desk with my other cataloging tools, along with our local plan for how we will assign the call numbers for the various volumes. This saves us a lot of “volume 4 of what?-style headaches!
This particular volume is volume IV of Epistolae Karolini, and volume 6 of Monumenta Germaniae historica Epistolae, so we give it the call number:
DD3.M8 ‡b E31 t.6