Weapons of the Romans; Michel Feugere

[Tarbicus]

His discussion of shields is interesting. He theorizes that the curved cylindrical scutum was not in universal use by legionaries, but was used for specific siege situations. He points to many depictions of legionaries (including some on Trajan's Column I think) carrying other types of shields.

That's cropped up before, and I'd never discount the idea of one type of shield per legionary.