Manufacture comes from manus, a hand, and facio, I make. Everything in Roman times would have been manufactured by individual artisans. The commonest armour worn by legionaries as well as auxillaries was mail, even after other armour was introduced. After a battle individual legionaries would have scoured the battlefield, picking up weapons, helmets and armour from dead soldiers, keeping anything that was better than their own. I suspect the only expendable bits of kit were the scuta (shields) and I suspect that the only uniformity was the design painted on individual scuta depending on century/cohort or legion. The sort of uniformity seen at Trooping the Colour I would imagine was most unlikely other than for elite units on special occasions.
happy modelling,
Mike