Many missions flown by the 15th gave a bomber crew double credits (2 mission credits) for the longer missions which would count toward their 50 mission tour. This link has a writeup of a mission flown 31 May 1944.
https://455th.org/Missions/May%201944/PDFs/May%2031.pdf
On page 6, (a summary of a "2 credit" mission) is the notation that the crews who returned who were classified as a "Prior Return" would get a single mission credit. Early returns got no credits.