Impression questions

[Caius Marius]

It is entirely possible that centuriones came again to depend on tinned or silvered helmets for field identification, possibly embellished with enamel or the forerunners of the settings found on some late Roman helmets.

Again? When did they rely on it before?

I read something on that in Dan Petersons book, The Roman Legions in Color Photographs.

(page 42)" ....During the Roman Republic the vast majority of helmets were bronze, so those of the centurions were often tinned or silvered for smartness or recognition. (Perhaps in the Principate, when bright, silver-like iron helmets became commonplace, some centurions may have preferred bronze, to remain more conspicuous on the field?)"

While I have heard that some of the info in the book has been found to have become outdated, is that the case with the above bit?

In the pic that the above quote is the caption for it shows a centurio wearing an imperial gallic "I" brass helm with a transverse crest. Makes me curious what type of, if any, modifications had to be made to mount the crest?