The TWSBI Diamond 50 Ink Bottle in Blue features a study clear tapered glass base with an anodized aluminum blue colored special top insert that is designed especially to fit on the TWSBI 580 and TWSBI Mini fountain pens, to make filling these piston filler pens cleanly and efficiently. If you remove the special filling top insert from the mouth of the inkwell, then the inkwell can be used to fill any bottle-filling fountain pen.