According to the Winchester-Western Centerfire Handguns Ballistics Chart from that time period the standard load was a 158 grain lead roundnose bullet at 900 fps from a 6 inch barrel.
Remington had it listed at 855 fps .
These were the standard loadings. There are others listed , such as Hi-Speed a 158 grain lead (no bullet shape is listed) at 1090 fps ! I'm not sure if these loads were intended for K frames , possibly just N framed guns .
For the older M&P's I would stick with factory ammo or reloads in the 850 to 900 fps range with lead bullets either round nose or semi-wadcutter.
I have a fixed sighted model 64 1970 vintage and the sights are regulated for 158 gr. lead bullets(RN or SWC)loaded to between 850 - 900 fps.
As for bullets I favor SWC, but I was a reader of Elmer Keith and he wrote they was the best....I do a lot of target shooting and cast and load a lot of wadcutters.
Gary