Helmet Boss - Acanthus

[PeterMc]

A query for the helmet fittings experts please - This is the 'acanthus leaf' boss from a Dublin Metropolitan Police helmet. It is the first time I have seen one disassembled, as they are usually seen with the 'ball top' finial. I notice that the decoration on the boss extends up to the very top, which is usally hidden by the finial.
It is also unventilated, which seems unusual.

Does anyone know if these 'acanthus leaf' patterns were used on other helmets, police or military, and if so what would the other fittings have looked like? Did they allow for the additional decoration to be seen?

Any ideas as to when these were first taken into general issue (again police/army if relevant)? I am sure this must have been a general pattern and not one specifically made for the D.M.P. - or am I wrong?