The TWSBI Diamond 50 glass inkwell with metal cap can be filled with your favorite ink and be used to fill your TWSBI or pens of most other brands.
The inner reservoir fills itself when you hold the bottle up side down (with the cap on). This allows for easy filling of your pen. TWSBI pens can be filled even easier. You can take the nib off and directly attach the pen to the tube. Converter scan also be filled that way.