The TWSBI Diamond 50 Ink Bottle in Blue features a study clear tapered glass base with a blue anodized aluminum top insert that is specially designed to fit on the TWSBI 580 and TWSBI Mini fountain pens.
The insert allows for a much more clean and efficient way to fill these piston filler pens. If you remove the filling insert from the mouth of the inkwell, then the inkwell can be used to fill any bottle-filling fountain pen. The cap of the TWSBI Diamond 50 inkwell will fit on the bottle both with or without the special insert.
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