The great majority of American-made Watermans from around 1917 to 1930 were marked on the end of the barrel with a numeric code that clearly identified the model. The basic system is easily learned, and runs as follows:
Older pens used a slightly different numbering system, which was used up until 1917 (imprinting of the new numbering system probably began in spring of that year, with a few months' delay before the new numbers first appeared in ads). The key differences in these older pens' markings are as follows:
It should also be noted that stamping the full number only became standard practice in the fall of 1908. Earlier overlay pens will often be found stamped with two-digit codes.
There were some irregularities in this system (e.g., the #20, an eyedropper with a #10 nib), but things really started to get confusing with the introduction c. 1927 of the #7 and the #5, followed a couple of years later by the unnumbered Patrician and Lady Patricia. For a while a modicum of consistency survived: the #32 and #92 had a #2 nib; the #94, a #4. Then the #32 became simply the #3, with the last overlay pens being numbered #403 even though equipped with a #2 nib.
Note that an entirely different numbering system was adopted in the 1940s for British-made Watermans, and that while US-made pens with overlays intended for export usually had a barrel-end imprint with no digits in the hundreds or thousands place, such designations did appear in catalog listings. In a 1912-dated French catalog in our possession, solid 18K gold overlays are indicated by a 7 in the hundreds place; silver, by an 8; and gold filled (doublé or) by a 9.