Transverse Crests, when did they start being in use?

[drsrob]

No metal remains of crests have survived

Ah, but some think that Vegetius was referring to the metal crest as seen on the Intercisa IV (e.g. Intercisa, Venlo, Gutmann collection) or the Richboough I (e.g. Richborough, Linz monument) helmets. Since most if not all Late Roman helmets were silvered, these crests would be silvered as well.

Possibly, but these helmets were wholly silvered and Vegetius specifically says "part silvered" if I'm not mistaken.