(Photo courtesy of Orbis Catholicus)
You'll note, of course, that the diaconal stole is worn outside the dalmatic rather than underneath. (They also wear the collarin around the neck and upper back, but that is not our point of focus here.)
The explanation for this is not that the Ambrosians developed their own form of vesting a deacon distinct from the rest of the Western liturgical traditions; it is rather that they retained an earlier custom.
The Catholic Encyclopedia notes that "...after the tenth century it was only in Milan and southern Italy that deacons carried the stole over the dalmatic, but at an earlier date, this had been common in many parts of the West."
Further to this, Archdale King notes in The Liturgies of the Primatial Sees that "the stole of the deacon is worn outside the dalmatic, as in Spain and Gaul before the Carolingian reform. It probably at one time hung down to the ground, and was always white in colour. The first council of Braga (561) specifically directed the stole of the deacon to be visible, in order that he might be distinguished from the subdeacon." (p. 385)
What we see here then is an interesting remnant which testifies to an earlier liturgical custom.
