This thread covers the GR257, I have yet to find info about the 267 though, and I'm racking my brain trying to remember what bike I saw that used the same type of seat clamp bolt from inside the frame triangle. I know it's a copy of some larger brand but really can't find which.

https://chinertown.com/index.php?topic=7368.0

@Jarkko2410

Which mud guards are those?

Could a front mud guard fit between tyre and fork?

Your bicycle looks really excellent. Versatile, fast and comfortable! Almost exactly what I picture my build to look like!!