Hi Michael,
I'll see if I can dig out my copies of the RNAS Communiqués but, as far as I can recall, on occasion a report from one evening can end up in the subsequent day's reports. (Also, if he were flying with any of the 5 Group RNAS attached to the RFC then their reporting times would apply).
I've also a copy of the 'Hun Book' somewhere. There are lots of duplications and also a few cases where claims are noted that were ultimately overturned.
However, on Collishaw's RNAS officers' file they've transcribed the claims into his service record, noting which ones were associated with the DSO and DSC awards.
https://discovery.nationalarchives.g...ils/r/C9748685
(Unfortunately it seems to run out of space before they get to mentioning December 1917).