San Giovanni in Laterano

"Giessen Anonymous"  a German cleric/layman from Mainz?
Letter to an unknown person

"Serenitati vestre"

Giessen, Universitätsbibliothek 1105, fol. 59ra-60va

Stephan Kuttner and Antonio García y García,  "A New Eyewitness Account of the Fourth Lateran Council,"  Traditio 20 (1964) 115-178 at  124
n die igitur beati Martini inchoatum est concilium.  Dominus papa apud Lateranum in ecclesia Salvatoris que Constantiniana dicitur summo diluculo, intromissis solummodo cardinalibus, archiepiscopis et episcopis, eodem die missam inprimis celebravit. 
The council began on the feast of Saint Martin (11 November, 1215).  At the break of dawn, the Lord Pope celebrated mass at the Lateran in the Church of Our Savior, which is called the Constantinian.  Only the cardinals, archbishops, and bishops were present.