I am Posta Tutta Porto di Ferro Dopia (Full Iron Gate guard doubled), and I am good in armour and without, but even better in armour than without it. And with such a guard I cannot use a dagger.
Similar to Posta Tutta Porta di Ferro, however, this posta braces the lead hand, allowing greater force and safety when jammimg or sweeping aside incoming attacks. Note that the masters lead side faces the players weapon. i.e. a right handed attack is defended by bracing your own left hand.
Bracing the arm in this way will make for much closer techniques. Jamming an attack with your left forearm would almost instinctively lead to you attempting a strike with the right elbow. The close proximity leads to this posta working better in armour.
Also, your left hand is incapable of doing anthing useful with a dagger here, and your right hand is busy. You cannot use a dagger of your own with this guard.