I think that adding the "nail" to the top is a bit of artistic licence. But a small spike is historically accurate, as this illustration shows (the guy on the horse has a small spike on his Spangenhelm). I have other pictures which show this small spike, or even a small boar of metal on top. Byzantine helms often had a ball, and helms provided with a mount for a crest are also represented in the artwork of the period. There are almost no existing helms left today, and artwork is about all we have to go by.