kopsy: I agree it's a strange situation. Maybe someone else will jump in with an explanation, but we're all working from the same book and, unless I'm missing something, 3D just isn't there. Could be a typo, I suppose, and 1979 should really start with 3D instead of 4D, but I have no reason to believe that's the case. You could make a call to S&W and see what they have to say. My understanding is they're happy to give you the production date if you quote them the SN. If you do that, come back on and let us know what they say.
swsnub: late 1982.