TWSBI Mint Blue piston filler fountain pen 

The ECO-T has a more pronounced grip section. Cap and end cap also have a rotary triangular shape design. 

To fill with ink: unscrew the end cap, immerse the nib and feed into ink, screw back the end cap, clean ink residue from nib and section

Nib are available in EF, F, M, B and Stub1.1

Pen cap will post onto the back of the pen

Pen cap has an inner cap to insure a perfect seal to keep the nib from drying out when not in use.