The Precision FP comes with a piston filling mechanism and has all detachable parts. TWSBI felt that it was important to allow the user to disassemble and reassemble the pen and completely experience the traditional aspects of owning and using a fountain pen.

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?)