Area Q was an open-air enclosure with a mosaic floor, west of the Red Wall, reserved for the inhumation of the deceased. In Q, a small drain was discovered. It collected the rain water and passed under the steps and floor of the clivus. Some stamped marks of 146-161 AD, made in the tiles that sealed the channelling, have allowed us to establish a basic chronological period for the construction of enclosure Q, of the red wall and the trophy of Gaius.

From: J. Toynbee - J.W. Perkins. The Shrine of St Peter and the Vatican Excavations, London 1956
Tombs R and Q, and the clivus and its eastern wall (the Red Wall), form a closely related group of structures, of which all except the first-named (R) are certainly of one build and constitute a single chronological unit. The drain, the purpose of which was to drain the open area Q, was partly built of tiles; and no less than five of these tiles bear an identical stamp, which can be dated between the years 147-161. Since there is little likelihood of five identical tiles have been reused from some earlier structure, we can be reasonably certain that this group of tombs was built very little, if at all, after the death of Antoninus Pius in 161. This dating has very important implications for the history of the shrine of St Peter.

Sources
P. Zander. The Vatican Necropolis, in "Roma Sacra", 25, Roma 2003
Margherita Guarducci, The Tomb of St Peter, Hawthorn Books, 1960
John Evangelist Walsh, The Bones of St Peter, New York, 1982
J. Toynbee - J.W. Perkins. The Shrine of St Peter and the Vatican Excavations, London 1956
Michele Basso. Guide to the Vatican Necropolis, Fabbrica di S. Pietro in Vaticano, 1986