Around '91. I also have one that's BNBxxxx and when I called S&W they told me Nov.'92.
I also have a 60-7 which is identical to the 36-7 but stainless.
The -7s were the last .38Spl versions before they got chambered in .357MAG. They got wider front/rear sights, were factory heat treated for +P ammo, and were the last ones to still have all tool steel, not MIM, parts.
IMHO the Dash 7's are the BEST of the M60's and M36's for the reasons stated above! As a carry gun the M60-7 has everything you want and nothing you don't.