I considered the military possibility almost immediately, but these marks were set in random areas such as alongside highways and in residential neighborhoods, and nowhere near any installations AFAIK.
The marks that I questioned are all CTDT survey disks that were set in the early 1980s. Since they were bluebooked with NGS, there are datasheets available, displaying all the usual information right down to an exceptionally detailed to-reach description, then finally the reason why the sheet is unpublishable.
I think Z15 has it right that the state of Connecticut set these marks for CTDoT business exclusively, in spite of the NGS bluebooking. Perhaps the bluebooking is merely certifying the quality of the monuments
I might go out and find some of them anyway, just to embrace the challenge. The only downside is that I can't log them on Groundspeak. Of course, there are also plenty of NGS marks out there that I can log!
Thanks for your help, guys!