OK, so something strange happened to me. I've scanned every one of the "original" 15 obelisks with every combination of two artifacts, except for duplicates, as well as with every single artifact. I collected 13 data points from those scans:
Historical 8, 10, 11, 12, 14, 15
Language 8, 12, 14
Biological 7, 9
Technology 7, 10
I believe these are the same ones that have already been found by several CMDRs. I've been making a spreadsheet myself and it appears to be consistent with CMDR Berzergker's spreadsheet, with some differences in the combinations used. I cannot say with 100% certainty that my spreadsheet is correct because of delayed/disappearing messages, but I think I have been able to account for all of those by also checking increase in the payouts.
HOWEVER, I was just returning to the site after a CMDR was nice enough to blow my SRV up due to some heated discussion, and while in Orbital Cruise I received a mission update with Decrypted Data Historical 9/21. I have no idea what obelisk it corresponds to, and the last set of scans I did was with two Caskets in my hold.
Edit "This data contains details on the weaponry the Guardians developed during the various conflicts that swept over their civilisation..."