This is a great question! I apologize for the late response; I have had quite a few questions lately, and I want to give them all the time they deserve. Yours is a particularly interesting one, in part because it touches on my favorite figures in the history of armour - the armourers themselves.

Part I - Milan and the Missaglia

In general, the early development of the armour industry is shrouded in anonymity. We have a few named armourers, and we know that there was already a vast armour industry in 13th century Milan, but we can't associate names with actual armours until the 15th century, when marked armours can be matched with our surviving records.

This means that most of the development of armour in the Middle Ages cannot be associated with any one person, family, or workshop. This includes the development of full helms, the development of plate armour first as a supplemental and then as a primary defense, and the improvements in armour's metallurgy. In these cases we can sometimes speak of the regions or even cities where some new technique of production or form of armour was introduced, but we can't get more specific than that. For instance, we know that by the later 14th century the armourers of Milan were 'slack quenching' armour - heating after it was done being shaped and then plunging it into a hot liquid like hot brine, molten lead or hot oil, or interrupting a quench in cold water before the steel fully cooled. This process transforms the crystalline structure of steel so that it is harder, but not brittle. We also have at least one example from England that is heat treated in that manner, but most heat-treated examples in the later 14th century are from Northern Italy - Allan Williams suggests that the practice developed there, and the procedure for slack-quneching plate armour perhaps remained as a kind of trade secret (though quenching was performed for other steel items, so it is not as though the technique in general was unknown). The Milanese were also associated with other developments, like the full breastplate replacing the 'pairs of plates.' This covered breastplate in the Bayerisches Nationalmuseum shows all of these developments - it is heat treated, seemingly with a slack quench, and despite its appearance the breastplate is a single piece of steel, with the fauld (skirt) attached to a backing.

In the 15th century we have both armourers' marks and surviving records, so we can match surviving armour to names. The name that looms largest in the mid-15th century is the Missaglia, the great armourer-merchants. I can't point to an innoation that the Missaglia made in the form of armour itself, but they did leverage the existing, contract-based system of Milanese armouring to create a vertically integrated, international armour manufacturing and trading cartel whose operations spanned the Mediterranean and beyond, including satellite operations in Naples, Barcelona and Lyon and clients as far away as England. They also owned their entire supply chain, including bloomeries, hammer-mills for turning blooms into sheets and polishing mills for polishing finished armour-pieces. I write about the Milanese manufacturing system here.