If the grips are serialed to the gun, then they are no doubt correct. The diamond was deleted in 1968. S&W did have a habit of mixing the old with the new to meet the demands of orders. It is entirely possible that your sqaure butt frame was made prior to 1968, but the gun was not "gripped" and then blued until afterwards. Without the diamond grips, it shipped after 1967.
One thing.....It looks like you have a rear sight with the vertical serrations. When I first saw that on mine, I thought "Oh no.......a Rossi sight on a Smith...." After some research, I learned that the Model 34 came with rear sights in both vertical and horizonal serrations. The rear sight is correct.
Enjoy the revolver! You got a good 'un!