All passages from the fragment are from the Contra of the Trent 91 Gloria in the Mass DIAMM no. 81 in the online edition, a three-voice Mass described in Leverett 1993. The Mass is in Mitchell's online edition of Trent 91 (see DIAMM Resources: Music editions).
There are 3 snippets of music in the available photo. The first two are at the top of the page with the second one upside down. The third snippet faces upwards and is in the bottom right-hand corner.
In the first snippet there are 2 part-staves. These give the Contra of Gloria 75,2–83 on stave 1 and 117–129 in the same part on stave 2.
In the second upside-down snippet are three staves. These are from three passages in the Contra of the same piece; 31–37 on stave 1, 67–74,1 on stave 2 and 107,2–116 on stave 3.
In the third snippet at the bottom of the page are 2 staves. These give the Contra at 38-45,1 with a part-legible ‘Glorificamus te’ incipit, and a part-legible Contra 74,2-81 due to page cuts. It looks like there is a paste-down in the bottom left corner. What is under or on the back of this a matter for speculation.