I'm back with what's probably the last update--After reading more posts on this site and staring at the seattube for a while, I began to think the 26.8 seatpost indeed was too small. So I (carefully and slowly) spread the ears of the seat cluster with a big screwdriver until the seattube cut was even from top to bottom, which also caused the small ridge below the cut where the steel had bowed out slightly around the seatpost to flatten out. After doing that the 26.8 post was clearly too small, so I stuck my 27.2 mountain bike seatpost in there. Voila, it fit! So it looks like the seattube is 27.2 after all, and the guy I bought the frame from stuck a crappy wrong-sized Kalloy post in there.

Also--there is no serial no. that I can find, but the frame size (54) is stamped on the right side of the bottom bracket. So I'm guessing it probably is a custom build like many of you folks have said. Either way, it rides great and I'm keeping it!