The serial number will probably start with an NA or an NB and it will be stamped into the underside of the frame in letters about 1/8" tall or less.
This machine was one of the first of the slant needle machines and it's good for straight-stitching and free-motion quilting.
What they're worth is hard to say - pretty much whatever someone is willing to pay you. :) It's going to depend upon what part of the country you're in, how many vintage machine fanatics there are around you and how many vintage machines are commonly available. :)