The TWSBI Eco has recently been released after years of development and although I do not yet own it, I'd like to share some information about it that I've gathered from others.
The TWSBI Eco comes in either black or white with a friction fit nib and feed available in extra-fine, fine, medium, broad and a 1.1 stub italic.
Similar in size to the 580 and only about $28, the Eco is TWSBI's most affordable fountain pen. It comes with a TWSBI wrench to disassemble the piston mechanism and silicone grease to lubricate the piston seal. The Eco also features a cap insert to keep the pen from drying out if ink is left in it.
According to Brian Goulet, founder of the Goulet Pen Company, the Eco feels solid and is lighter than the TWSBI Diamond 580.
The pen can be posted easily thanks to an O-ring that keeps the cap on the barrel. A feature that the 580 and Mini don't have.
I predict that the pen will sell very well and will become a very popular beginner's pen thanks to the low price, piston mechanism, and the wide variety of nib sizes. It will also be an attractive option for people wanting to try piston filling pens as it's one of the most affordable.