Mine is identical to yours in condition and is serial numbered 3D163XX with inventory number 21441.
At maximum strength the Detroit Police Department had around 5,000 members in the late 60s and into the early 70s.
Those two inch model 10s and Colt Detective Specials were issued to the Command Officers and plain clothes Detectives. They came in nickel and sometime blue. Uniform got 5 inch S&W model 10s or Colt Official Police in nickel finish.