Let's see, a salt free 20 gauge as you describe, probably 26 1/2", square knob, short tang, should sell for about, as you say $1800 to $2000. A similar, minty, long tang, round knob gun in 20 gauge, 26 1/2", is a $3000 plus gun today if all original including butt. The square knob and short tang means something, but the stigma of possible salt is the major impediment.