Probably the best known tubular nib pen is the Rotring 'Variant'. This photograph shows a set of three Variant pens and a holder in a case. Each pen unit is for a different size of line and has its own ink reservoir. The red tool is for removing the nib from the pen unit for cleaning.
The same type of nib unit was also used in the 'Rapidograph' fountain pen, an example of which is shown below.