My latest editon of Standard Catolog of Smith and Wesson only covers up to 2004, so I'm guessing your 686 was made after that. However, a call to S&W at 1-800-331-0852 (Inside USA) 8:00 a.m. to 8:00 p.m. Eastern Standard Time (Monday - Friday) will get you a DOB free of charge.
Thank you, will do.