Waterman used 4 digits (XXXX) for their pen designations. Yours is a XX12. The first two digits indicate any kind of metal overlay work (e.g. 0512 would have a gold-filled overlay, 512 would be solid gold overlay). The third digit is the model. A 1X is an eyedropper. Eyedroppers were early models, and there was some lag in the designation when Waterman switched to lever fillers, so some leverfillers are also #1s. Eventually that was changed to XX5X. The last digit is the nib size. In your case #2. There may be some different designation because of the gold plated bands, I'm not sure how they designated early eyedropper trims. Other markings could include a "1/2," meaning the pen is thinner than usual, or a "v," meaning the pen is shorter than usual (for vest pocket).
Check the butt of the pen. There should be stamping indicating the model number. It may be highly worn though.
I used "X" as a placeholder to show how the numbering system worked, that's just to explain it. There was actually a later pen called a "52X" that was a model 50 with a #2 nib but with a larger than normal body.