What does this icon mean on Folders Properties dialog box?

[T Maynard-Smith]

I am looking at the Properties box for a normal folder, e.g.:The Read-0nly Attribute cycles through the following three options when clicked: The second one (a tick) presumably means "Read-only". Normally I would expect the first one (blank) to be the opposite, i.e. "normal" or "not Read-only" or "Read/Write". But if one clicks to clear the box and then Apply or OK, it comes back as the third option which is completely non-obvious. It is also easy to mistake for the second option if one is not expecting it. What does this third option mean? If it is just "not Read-only" then why no

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