Professor Dobszay comments that the 'Tridentine Rite' is in fact a cut-down version (for the use of the Roman Curia) of the true Roman Rite which existed throughout the West in various forms. This clip will show you what he means.
But first, the 'Epistle' (Malachi 3:1-4). Records show that the Epistle was sung from all sorts of places, including from lecterns at the footpace of the altar. Some texts decree that it is to be sung from the Rood Screen, and hence many commentators have commanded their subdeacons (and deacons for the Gospel) to clamber up to the rood loft to do so. A simple look at almost all existing rood screens will tell you that this is ridiculous. Access is almost always by a tiny door and spiral staircase, and the idea of ascending this in vestments, still less with taperers and crucifers and thurifers is absurd. Maybe in Salisbury Cathedral it was possible, but since Wyatt removed the screen in the 18th Century to 'restore' the Cathedral, we can no longer tell. More likely, it could have been read from the door in the screen, assuming people to have been sitting in the nave. But this, too, is unlikely, since I am pretty certain that people did not generally, and certainly in large numbers, occupy the Nave of great churches for High Mass as they would today, (though parish churches might be another matter). Consequently, Epistle and Gospel could be and were read almost anywhere (the Gospel always facing north, of course). We used the lectern, which conveniently could be turned to face north when required for the Gospel. You'll see the rulers doing this at the end when they intone the Sequence. I strongly suspect that this is what was done in the 16th Century in Merton Chapel.
The Subdeacon, then, sings the passage from Malachi: the chant is identical to that in the Roman Use. At the end, the taperers return accompanying the Acolyte, who is carrying the corporal, chalice and empty paten wrapped in a humeral veil. The subdeacon follows on, carrying the Epistolary on his arm. The Acolyte takes the chalice to the credence, and the humeral veil is removed. He then takes the corporal to the altar, where he leaves it, kissing the altar as he goes. Meanwhile the Celebrant, Deacon and Subdeacon read the Gradual at the sedilia. The Deacon then goes to the altar and spreads the corporal. The Subdeacon goes to the credence, and pours in wine, then carries the water to the Priest to be blessed, then pouring a drop into the chalice. He then veils the chalice and returns to his seat.
Which is why Graduals are so long in the Roman rite; they were intended to cover all this (or similar) action which had been dropped out of the Roman Use by the 16th Century.