I have a few Parkers being the same model with only one of them being stamped (on its feed, not the nib itsef) as "Fine".
I inspected them one by one to find out how the pellet size on that one nib compares to the pellets on others. And if my theory is right I have some XF, some F and one M nib. But I wonder if the pellet size really has a 1:1 impact on the nib's width. Has it?
So is this a good method or do I need to compare the actual lines drawn by each of them (and dip the 2 MINT ones...)?