The TWSBI Diamond 50 Ink bottle has been designed for every type of fountain pen.
It owns a specific upper cap design for Diamond , 530, 540, 580, Mini fountain pen.

The TWSBI Diamond 50ml ink bottle is a glass bottle with anodized aluminum top; 
the perfect accessory for any fountain pen addict! 
The bottle allows for easy refilling of your fountain pen by offering an upper reservoir 
that fills with ink when you turn the bottle upside down (cap on of course!) 
The reservoir makes it so you will not have to dip the nib into the bottom of the bottle when the ink gets low. 

As an added benefit to those who own the Diamond 530, 540, 580 or Mini fountain pen 
you can use the colored band attachment to refill your TWSBI fountain pen without having to use the nib at all!
Simply unscrew the front section of your Diamond fountain pen and attach it to the filling apparatus.
Turning the piston knob of the fountain pen will draw the ink up into your pen. 
An amazing functional and aesthetically pleasing fountain pen accessory from TWSBI