"261E" seat tube on an old bike frame I've bought an old second hand bike frame and I'm trying to build it up into a road bike. Things have been going quite well, but I've got a bit confused by the seat tube. At the top of the seat tube in the frame, there's the text "θ 261 E" (it might be 264 - the "1" is a little bashed up). I assumed that meant I needed a 26.1mm seat post, but they seem impossible to find.

I also used a pair of calipers to measure the inside of the tube and it seems to be about 25.5mm (presumably it might squeeze down to less).

Does anyone know what this marking means, and what size seat post I actually need?

Many thanks.
Well, the seat tube is definitely 25.4" (thanks for the help, Criggie): I've bought one that seems to fit. I still don't know what the stamped text means, but I guess it doesn't matter.