With A Kalloy Plain seatpost on both bikes and a separate saddle clip, ( Brompton's ($30) Pentaclip is the best)
you can slip the saddle clip off the post top and move It.





yes standard seat post , and frame bores are Increasing in even Increments, 0.2mm at a step , so the 26.5 is likely a measuring error.

USE [UK] precision machines their seat post sizing shims, they made a 25.0 seat post and a 27,2 seat post and sold shim sleeves to use the smaller

seatposts in all the sizes in between.

[ The Shim thickness difference is 1/2 the difference in diameter , 26,4-25,0= 1.4 , so shim stock thickness has to be 0.7mm ]

Might need an accurate digital caliper, they are not that expensive with China making them, too.