Difficulty Sourcing a Chant

A friend of mine is doing a project about chant accompaniment and is struggling to find the source of the attached Panis Angelicus "alter tonus" chant. I went and checked the usual books as well as Global Chant, Cantus, GregoBase, and Google, searching by both text and musical incipit, and have not found anything either. Does anyone recognize it, or have further suggestions?

Thanks very much!