I've been looking at Luke Ueda-Sarson's excellent Notitia Dignitatum site, which is the site for the Late Roman shield design alficionado.
The good news is that there is a similar design to my other unit. I hesitate to copy images from the site due to copyright, but the Gratianenses have a very similar shield design to what I thought were the Leones Iuniores. It turns out that, due to the failings of a copyist back in days of yore, the names of some of the units most likely have the wrong names. My Leones Iuniores are, in fact, probably the Leones Seniores. The Gratienses probably aren't the Gratienses, either, but someone else! It is all fearfully complicated. However it means I can retain both units, and can probably "fix" all of the shields in an evening's work. Tomorrow evening's work!