Welcome! With the agency stamping, the chances are good that a letter of authenticity will give you some history on it, namely its ship date and destination:
Smith & Wesson Historical Foundation - Letter Process - Insuring that the rich history of Smith & Wesson will continue for generations to come
As far as a value, the condition is the most important factor - figure $400 for a well-worn example, up to perhaps $550 or so for a really nice one. Its law enforcement connection may interest some but generally won't have a great influence on how much it is worth unless someone collects guns from this agency specifically and is willing to pay a premium. Hope this is helpful.