This is a question of no great moment, just the fruit of idle contemplation; of the various point sizes Waterman produced, does anyone have a clear idea why they went to the bother? Purely for appropriate scale to the pen body? Some kind of implicit durability (a #5 has more mass to soak up punishment than a #2)?

You need to go back to the 1880s and look at how pens were sold. If you look at pen ads and catalogs in the late 1800s, it is all about the pen (nib in our contemporary parlance). Most pens were dip pens, and holders were pretty much the same (well, many differences in trim etc., but not in function). Most pen companies made a variety of nibs with sizes 1-10 or so (which were not consistant from company to company, of course) and buyers would pick the size nib that felt best to their hand. So when many fountain pens came on the scene, they took that model and ran with it. Waterman carried nib sizes 1-8 (plus a 10 as a specialty item) because that is what customers expected.

It is not just a matter of appropriate scale to the pen body, though that does play a role in the 15/55-20 size pens. However, the 12, 13 and 14 are all the same pen body that differ only in nib size. The 52 and 54 are likewise the same pen with different size nibs. I believe there was also a 52X, which had a #2 nib, but a 55 body (for greater ink capacity).

In practical terms, there is a difference in how different size nibs flex (and we are talking about early pens here, so flex was important).

What is interesting about Waterman is that they built their pen models essentially around the nib size up until the 1930s, which seems later than most other companies. I think Sheaffer had restricted their nibs to just a few sizes (3, 5, 8?) much earlier and by the time the balance came out they had pretty limited model-nib combinations. I am not sure Parker was even putting sizes on their nibs by then. Waterman did cut it down in the late 1930s, with the Model 7 (using a #7 nib that was about the same size as the old #5), and the various 3-series pens (using a #3 the same size as a #2), and the 100 year pen, but they still had the 94, 92, the 5 (with a #5 the size of an old #4) and the 5X series available during a good chunk of the 30s.

It is also interesting that at least some of the 2nd tier pens (thinking of the Sears brand pens from the late 20s and 30s) also sold various models by nib size, though the size always corresponded with the size of the pen and so was an indicator of that. Still, 6 different nib sizes, from 1-8 (as shown in my avatar).

John