Hello Zach, thank you for the background info on this rare and interesting series. Actually, I saw your coin on Gert's site and just drolled! You do have ambitious tastes and good luck with your passion. I too enoy this series and have restricted it to two catalog sales;FRANK STERNBERG AG-ZURICH Auction XXXIII September 18 and 19, 1997
25 examples pictured with details! Also check out Numismatik Lanz Auction 100 November 20th, 2000, which has pictures of 5 including choice examples of Valentinian, Valens and Gratian. The old Cohen work also is a OK reference and is available online.
The real reason of this reply is to recommend the book by Andrew Alfoldi (translated by Harold Mattingly) titled; A CONFLICT OF IDEAS IN THE LATE ROMAN EMPIRE: Clash between the Senate and Valentinian I. (1952)Oxford at the Clarendon Press. Chapters discussed; Pannonian Emperors, The Pannonians at the Cort of Valentinian I, Corruption and Its Antidote, Terrorism, The Clash between Valentinian and the Guardians of the Traditions of Rome, The Late Classical ideal of culture in conflict with the Illyrian military Spirit. There is also an Appendix entitled The Emperor after the Senate's Heart in the Historia Augusta.
Gives a clear picture of the dynamics during the period your series was minted.
Claims there is an earlier volume that dealt with 312 AD onwards and was expected a third volume with magic as the main topic. Have not searched these out as of yet.
I found this book at bookfinder.com, but it may be available via an university library.