What do nib numbers mean?
Look on top of many older nibs and you will find at the bottom of the imprint a number. This number was used to denote the size of the nib, not the thickness of the writing tip.
Unfortunately, there was no standard numerical scale for nib sizes, so while one can be certain that a Waterman #2 will be smaller than a Waterman #5, one cannot count on similar consistency from brand to brand. Some makers -- Conklin being the prime example -- changed the size of their nibs quite drastically over the years. Economy brands sometimes also engaged in a sort of nib size inflation, putting large numbers on nibs of quite moderate size.
Among the better penmakers, #2 was the most common basic size; #8 was usually the largest standard nib, larger numbers being used primarily for giant pens. John Holland's fountain pens used nibs running from #12 on up, but their #12 was the equivalent of a Waterman #2, their #15 the equivalent of a Waterman #5, and so on.
Numbered nibs began to fall out of favor among major American penmakers in the 1920s. With the shift to pens identified by name rather than number (e.g., "Lifetime", "Duofold", "Endura", "Balance"), nibs began to be marked correspondingly, and often did not bear any numerical designation of size. For a time both Sheaffer and Parker placed long serial numbers on their top line nibs, and from around 1935 Parker placed a numerical date code below the nib imprint. Nonetheless, many makers continued to use numbered nibs, and many used both numbered and unnumbered nibs.
Marking the thickness and style of the writing tip on the nib itself seems to have been a European innovation dating to the 1930s, which did not find general acceptance in the USA until the 1950s (some American special-purpose nibs were marked "ACCOUNT" or "POSTING" in the '20s and '30s, but this was exceptional). Sheaffer Snorkel and later Touchdown pens often do bear an etched mark denoting tip grade, but other later Sheaffers do not always bear such marks, and no indication of tip grade appeared on Parker nibs until the 1960s.