Your SN should have N plus 6 digits and 1984 is likely. I have one with the triple alpha SN from 1986. The advice to check GB closed sales is good although I'd throw out the highest and the lowest prices. Local factors can influence prices too. Ninety percent condition suggests quite a bit of blue loss and wear although percent condition estimates are very subjective. That said, I'd think a 90% -1 would be $900/$1000 or below where I shop. One in NIB condition perhaps $1200+. Last one I actually bought was about three years ago; a 6 inch with box/tools which appears unfired for $1050. Nice gun, enjoy it.

Jeff
SWCA #1457