Late Roman Shield Personalization

[Franklin]

Plus, I figured that the shields would be assembled somewhere blank and then sent to the units that needed them

No, that actually makes quite a bit of sense. We know that most arms and armor were manufacured in large-scale fabrica. Consider this, though. If you're shipping a large number of shields, you'd need to compress them to save as much room as possible. If umbos are already attached then it becomes impossible to stack them one on top of the other. If I had to guess, I'd say they were probably sent blank, like you suggeset, but with the umbos stacked separately and attached on site.

Just a theory, of course, nothing to back it up.