Probably 1967 is as close as I can get by trying to find ones near yours in the database because I don't have ship dates for many in that era. I have 520386, and it shipped on May 10, 1967.
At that serial number it most certainly has a contoured (not flat) thumbpiece. I should also think if your stocks are original to the gun (right one will have a serial number stamp on the inside), they are probably diamond magnas (large diamond around the screw and nut escutcheons).
Which finish does yours have (blue / nickel)? Which barrel length (2" or 3")? Which frame (round or square butt)?