This is surely for medievalist geeks. Reading through Eric Knibbs, "How to Use Modern Critical Editions of Medieval Latin Texts," History Compass 5, no. 5 (2007): 1521-49, I came across a link to web page with what seem to be all the abbreviations one is likely to find in a critical edition of a classical—I add: medieval—text. The page (here) is put together by Karl Maurer of the University of Dallas.

Maurer's page is probably overkill for the Thomist. Still useful—indeed, normative, perhaps—is Fr Antoine Donaine's «Liste des abbréviations latines et sigles recommandés pour l'apparat critique,» Bulletin SIEPM 2 (1960): 142-149, which you can see on Enrique Alarcon's miraculous Index Thomisticus website (here).