H.R.Robinson Helmet Typology

[Titvs Statilivs Castvs]

I think the most important groups would be the period indeed, wouldn't it?
That would come first in the name (1), then a type of helmet, as we now know the Coolus or Imperial Italic (but these could be other names as well) (2), then a letter for more specific characteristics (3), and to define exactly which helmet you're talking about, the finding spot.(4)

What do you think of this?

(1) would be early republican, late republican, early imperial, late imperial,...
(2) would be based on rather big similarities, such as now the Montefortino, Coolus, Imperial Gallic, Imperial Italic,... are used.
(3) would be exactly as it is used now.
(4) finding place, but this would not be included when talking about a certain type, only when talking about the find itself.

What other properties are important, or could be so, to classify the helmets?
Maybe infantry/cavalry can be added, but to divide them into auxiliary/legionairy would be extremely hard and probably just not correct; as we don't know who used it about all of them.