From innovative company TWSBI is their new Diamond 580 clear demonstrator fountain pen. A piston-filler fountain pen (traditionally found on more expensive fountain pens) in a modern design - all capable of being disassembled and reassembled by the user!
From the manufacturer:
A quick no brainer guide in choosing nib sizes in an overly complicated fountain pen world:
If you write small, go with an EF
For the normal writer, F is the most popular
If you tend to lean on a bigger font, M will be perfect
B is for statements!! Sign a check in B, write a header in B, do some coloring/shading in B
Stub nibs are italic in writing style (have a special card or invitation to write?)