f79r - something written under water ???

[oshfdk]

I don't think I see any clear letters there. Some lines completing the semicircle design above the paint, some plausible strokes (a sideways ^ that looks a bit similar to an r) and the rest looks more like the normal variation of paint density magnified by the contrast boosters.

I think taking either a UV image or a good close up with a modern phone may provide more information.