I bet this is old hat to someone on this forum: CPDL has two "O salutaris" settings attributed to Palestrina. Neither is listed in NG or the Opere; according to Marvin's Giovanni Pierluigi Da Palestrina: A Guide to Research, the first seems to first appear in Lück's 19c. Sammelung vol. 2 (the other was hosted at the aptly named server "ubisunt", which I'll have to try the Wayback machine on). What's not yet clear is whether this is a contrafactum, an uncertain attribution, or a 'spuriousity'.