Our friends at Little Hollow Customs make this rollerball adapter to allow you to convert your fountain pen to a rollerball by simply threading this into the section in place of a Bock 250 (#6) nib unit. Compatible with any pen that uses a Bock 250 nib unit. Internally, this uses a Schmidt PRS rollerball tip, which allows you to use your favorite ink in a cartridge or converter, with a rollerball tip! 

Pair this adapter with either a 0.5 (fine) or 0.7 (medium) nib in black plastic or chromed metal. Indicate choice in the notes.