Ishiwata MAGNY-X seat tube is 28.6mm O.D. with 0.9 walls at the BB end and 0.6 walls at the seatpost end. 28.6mm minus 1.2mm = 27.4mm I.D. Typically, 27.4mm I.D. seat tubes take 27.2mm seatposts. Because of manufacturing tolerances and heat distortion when brazing the seat lug, seat tubes should be reamed after brazing. My guess is that it has been pinched or bent by a ham-fisted P.O.

I'd take the frame to a a LBS and have them ream the seat tube to 27.2