Time was, when I found myself quite often explaining that in the Provinces of Canterbury and York (and probabably, for all I know, in other parts of the Anglican Communion), the Dismissal at the end of Mass had a double Alleluia during Eastertide; but that in the Traditional Roman Rite this usage was confined to the Octave of Easter. To be frank, I must confess that I regarded this as typically Anglican overdoing things. I may even have criticised it for failing to mark the important distinctions between the different parts of Tempus Paschale.
But ... stone the crows!! I recently had occasion to be saying Mass at a Carmelite altar (Holy Sepulchre observance) when my attention was drawn to the provision in the Carmelite Missal that ... the double Alleluias at the Dismissal continue throughout Eastertide!
A perfunctory flip through the two volumes of Jungmann failed to give me any more information on this. Did this usage survive in any of the other monastic breviaries as they stood on the eve of the Council?