I somehow remember my '88 Sirrus having a 27.2mm seatpost, or at least not being inconvenienced by it. I could still be remembering incorrectly. Certainly the carbon Allez had 27.2. My '85 Allez SE, as it was built (likely) by Miyata, sports a 26.8mm seatpost.
Some "xx.x mm" seatposts are larger than they say, and sometimes the frame opening is smaller than it should be, so it looks like that's the case with yours. Good luck!
Last edited by RiddleOfSteel; 10-10-19 at 12:39 PM.