The right shield shape selection

[Gonzo100100]

I have GBP Dark Age Cavalry set to paint and they have two variants of shields, one is in ‘0’ shape and another ‘o’ shape. My understanding is that if I want to make an army from IV century I should use ‘0’ ones but if I want X century, then ‘o’ variant is the one to go. Am I right?
Please correct me if I am wrong.

[AdamPHayes]

That sounds about right. I think you could get away with round shields for earlier cavalry too. What / when the whacky selection of helmets supplied are supposed to be is anyone’s guess...