Question about Sigg Shield One

Very tight but it does fit. You have to force it in and out. It can get scratched if the holder is metal.

The diameter fits, but the shape doesn't really. There is no "neck". However, there are metal bottles for bikes, e.g. this one:

Elite Syssa Race (0.75 l)

It does crack, but as it's unpainted it's not that annoying.

I have a plastic holder so the bottle goes in without any problems.

Yes, the bottle fits. But it has to be pushed in with a little pressure.

Probably for the water bottle holder. In vehicles, it fits in cup-holders, so it's a standard size.
/!\ the 1l version is bigger and doesn't fit.