Hello/Bonjour Martin,
Thank you for these additional details. It seems that the participation of the 305th Bomb Group is validated by an information (a link) that I just found this morning on the NARA website. There is an entry for the Group regarding an operation on the city of Bruz, located very close to Rennes (south southwest), but a city also very close to the Rennes/Saint-Jacques airfield. Link:
https://catalog.archives.gov/id/2874173
In the 306th BG report, I read that the target (the airfield) was covered by clouds and it is possible that the 305th BG then bombed Bruz; there was a fuel dump there - a target of opportunity for the Group?
Once again, merci beaucoup for your assistance.
Best regards,
Frederic