Yours probably shipped in 1999.
If you have the white outer box that housed the clamshell case, the “Spec Ord Code” on the label is a Julian style date code. The first character designates the year and the last three characters represent the sequential day of the year.
This 686-4 has a Spec Ord Code of
6121, which translates to the 121st day in 199
6.
If it has the envelope containing a fired case, look at the firing date. Both will be very close to the manufacture date.
S&W used “Scandium” as a trade name. The frames are aluminum alloy with traces of scandium added to the mix. Scandium is a rare earth element that substantially increases the strength of aluminum alloys. It’s much more expensive than gold by weight.