[Jack]

Dunno where you could look up the serial # of a 686...maybe S&W's website?
I can tell you the L frame series (the 686 was the first of the L frames) came out around 1980 or 81.
The L frame is a bit heavier than the K frame.
Traditionally, the Model 19 (K frame 357) was sold to a ton of police departments. Years ago, police departments would practice with 38 Special ammo, and carry 357 Magnum ammo as duty ammo- and the 19 was very, very popular with police departments.
As time went on, it became more common for police departments to practice with the higher powered duty ammo equivalent (357 magnum). The model 19 started showing some wear when fed a steady diet of magnum loads.
Smith and Wesson responded with the slightly heavier L frame to stand up to constant firing with magnum ammo, and in the 80's, tons of 686 were sold to police departments. So much so that for the first year or two of production, it was really difficult for a civilian to get a 686.