Not compatible for SOLO Ink Pen! 
ONLY compatible for SEPIA Ink Pen

International standard size 3.4mm bore diameter.
Sepia color, non-waterproof - can be washed out by water, but retain the visible line.
The package includes 10 cartridges.
Cartridge length: approx. 52mm; 
Cartridge Diameter: 4-6mm;
Bore diameter: 3.4mm