Can you help to identify these frames, bikes or parts?

^ Yep, 26.0 is pretty significant - it means the bike does not have 28.6mm OD tubing.

If the OD of the tubes is perhaps 28.0, and it's a plain gauge tube at 0.9mm wall thickness that would be a nominal 26.2mm ID so allowing for eccentricity a 26.0mm seatpost would fit.

28.0mm tubing tends to be referred to as "French"... Most columbus frames were 28.6, but Vitus were typically 28.0, as was the 531 for the french market.

The only other bikes I've seen 26.0 seatposts on are Japanese - I had a centurion with a 26.0 post - made from plain gauge Tange tubing.

There are obviously exceptions, today I'm riding a Cliff Shrubb track bike with a 26.0mm seatpost - Some UK frame builders used Vitus tubing and some used french 531.

Turns out I made a mistake and the seatpost is actuall 27mm. Stupid me, still: thanks for the lessons on tubing!