Update--the guy brought me his 6.0.6 disks to look at, but said that he didn't want to part with them at this point :-/

He did, however, say that if he ever did part with his Mac Classic, I would be the one who would have first dibs on it, complete with those 6.0.6 disks.

I got to take a look at the contents of the disks, and they're even more special to me because the creation date of the system file happens to be my birthday. They are indeed genuine 6.0.6 disks, and they could be one of--if not the only--set of these disks in the world.