SteveH may know how to tell a rare one from a non-rare one. I can't and neither can most sellers. But the coffin top cabinet does place it a bit earlier than a flip top treadle, or one with an electric motor. I've given up trying to get sellers to come down in price because their "really rare, from 1857" machine with a built on motor - isn't from 1857 no matter what some joker told them about the serial number. (BTW, the last patent date on this one is 83. . .)
I think a lot of novices get fooled by the look, and assume it must be really old and worth lots - but that may not be the case. the collector book I have says the style of the machine changed very little from the 1860-1870's up until they quit making them completely in 1946. This includes the patent dates on the stitch plate.
Oh my, I just searched eBay and there are several new listings for 1883 Willcox & Gibbs. One even says "post civil war era" and then goes on to say even the foot pedal (electric of course) is original Willcox & Gibbs. Well, I guess 1910-1920 is post civil war . . . though based on the foot pedal design, this one is newer than that.