Two variations: please check your pen! 
 
Sepia color, non-waterproof - can be washed out by water, but retain the visible line.
The package includes 10 cartridges.

For Solo Pen:
International standard size 2.6mm (7/64 inch) bore diameter.
Cartridge length: approx. 38mm (1 1/2 inches)
Cartridge Diameter: 4-6mm (5/32 - 15/64 inch)
 
For Sepia Pen:
International standard size 3.4 mm bore diameter.
Cartridge length: approx. 52mm
Cartridge Diameter: 4-6mm
Bore diameter: 3.4mm
Bore diameter: 2.6mm (7/64 inch)