Pictured from left to right
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26.0 x 320 mm -Uno Advance Project
26.6 x 230 mm -generic
27.0 x 180 mm -80's Fuji road bike (possibly 27.1 < 27.2)
27.2 x 300 mm -Kalin
31.6 x 260 mm -Bontrager SSR
25.4 x various lengths (not pictured).
PILLAR posts (steel and alloy) require a separate seat clamp assembly (clamps are $3) -
22.2, 25.4, (#out 25.6), 25.8, 26.0, (#out 26.2, 26.4, 26.6), 26.8, 27.2, 28.6
Very short posts <180 mm just $6.
Vintage American types for Huffy, Schwinn, Murray. 7/8", 13/16", 3/4" (~19 mm). $12