Congrats on a nice find! You have a late second model of the "Dick Special." The first series had half-moon front sights; earliest of which had square butts rather than rounded. The second series began after WWII. I have a second series example made in May of 1959, much the same as yours, but blued. The serial number on this one is 735xxx. Later in this series, sometime in the early '60's, the grip frame was shortened to the same size as the similar Colt Agent. The grips were modified so as to give the same overall grip length as before. The third and last series had shrouded ejector rods, starting in 1972. Yours is a second series with the shortened grip frame, so I estimate it was made in the mid- to late '60s.
I wrote an article on the Detective Special which appeared in Dillon's
Blue Press catalog/magazine in the November, 2008 issue. You could call 1-800-223-4570 and ask one of the sales reps if he'd get that back issue from the editor and send it to you.
Here's mine (with the longer grip frame) for comparison: