As I continue my exploration of flex, I am finding that some nibs flex just fine but are miserable to write with. That is, the quality of a nib is set by its weakest characteristic. Great line variation and good flow are not enough to overcome rough writing or the need for excessive pressure. So, we struggle with terms like "noodle" and "superflex", but it only takes a short trial to come to terms with the flex characteristics of a nib. Fortunately, writing a page defines a nib pretty well. Not so easy via the internet.
In particular, the pleasure a nib gives is largely dependent on smooth writing and that is hard to know without hands-on testing. Smoothness is dependent upon tipping integrity, equal tine length, tine alignment and tip shape. These characteristics can require high magnification to evaluate, and are rarely detailed with a nib offering. Therefore, online buying of vintage flex is, in my experience, a risky endeavor. Almost all of these nibs are worn. Some of them are just too worn to be of practical use.
Here's a case in point. The first two pictures show a Waterman's Ideal #2 with a good appearance, easy flex, nice wet flow, and line variation from F to about 1.3 mm. However, the tipping has pretty much shattered and one tine is longer than the other. Tip shape is ..... complex. The result is a rough and unpleasant writing experience. Sadly, this nib is toast. I can't justify the cost of retipping a humble little #2, even if I were not worried about what that process would do to line width and to the temper of the alloy.
Next, we have a 1st gen Wahl Doric (over sized) with what I think is a #5 Flexible nib. This flows well, and flexes from 0.2 - 1.4 mm. It does take a bit more pressure than I prefer but this is a keeper. Despite the obvious area of wear, the tines are of about the same length, the tipping has not shattered and there is a continuous band of tipping at the very end of the nib. Writes well.
Finally we have a little Mabie Todd #1 from a Blackbird. This nib flexes with minimal pressure, varying in the range 0.35-1.9 mm. There is a relatively small wear area at center, lots of smooth tipping forming a nice ball, equal tines and, as you would expect, this nib is a pleasure to write with. Now, if I could just find more nibs with this group of features.