Large Initial Letter Painted In Gouache On Vellum, "the Trinity," 15th Century.

Large initial on vellum, from an antiphonary, depicting the Son (Christ with his cross) on the left, the Father (the Pope in the center holding the golden triangle of the Trinity), and the Holy Spirit on the right, depicted as Christ holding the Dove.
All three are dressed in white robes, worn beneath a large red cloak. They are surrounded by golden halos. Blue clouds surround them, with three heads of winged cherubs at the top and bottom. Beautiful floral and foliage decoration on a gold background surrounds the lectern. 
The initial may represent the letter O.
On the back of the initial is a musical score from an antiphonary, with a Latin text. In the lower left corner is a man’s head in profile, drawn in brown ink and wearing a wide-brimmed hat, attached to a letter.
The entire piece has been cut flush with the initial.
France, 15th century.