f79r - something written under water ???

[Rafal]

Contrast-enhanced saturation channel from the old Beinecke scans.
I don't know what to make out of this: the circle at the top appears to be totally obscured by the blue paint, so it doesn't seem that ink below the paint can show up?

Beinecke scan is worse, Jason Davies is better to see these details.

I would say there is no circle at all.
Just a half circle about the water. It doesn't have its part covered by water.
But I agree with Koen, a multispectral scan could shed more light to it.

By the way, that's how I got the "letters" more visible: