The model 617 was made from 1990 through 1996. Stainless steel, 6 shot, 4 or 6 inch barrel (usually but not always with full barrel lug). Square butt with smooth fingergrooved stocks that came in different configurations. The 617-2 had a 10-shot aluminum cylinder, and round butt frame. Aluminum cylinder was not popular and changed to stainless steel in 1997. I don't find BNT listed as a serial number prefix - please check it again.