I have two Cross Townsend FPs, in that dark green/black striped pattern (looks fantastic, but don't know the 'official' name for the color), and both originally had medium nibs. I swapped out one for an extra-fine, and just tested it. The lines are the same width!
I must not be taking something into account. The only difference between the two is that the XF gives a more tactile feedback; you can really feel the paper you write on, while the M is more of a glider.
Thoughts?
Thanks all,
ScottT