That was my first feeling as well! Since I've never seen a 20th SR4 fretless for sale, I can't tell you from experience. But given that good condition fretted 20th with a beautiful top can go well over 1500, I wouldn't sell a fretless for under 1600. However, the current used market is pretty unstable and you could get one for less.
I think there were only about 20 - 30 fretless 20th SRs made in 1996.
Might be fewer - I have the exact numbers somewhere. I'd agree with the $1600 + pricing estimate.