Black Fly nailed it. The early M77 stock profile was slender & trim, with a very nice balance in the hand. Sometime around late #71 prefix (1975), the stock profile gained weight in the forend & grip area, changing the balance & feel of the rifle. Why this happened, I don't know! I've owned a #73 prefix M77R/.243 which had 3/8" of forend wood on either side of the barrel channel, making the rifle feel very clubby in hand. I gladly traded it for an early M77.
By #75 prefix (1981), the stock went on a diet & returned to a similar profile of the early M77's. So, when purchasing a used stock, take note of these features. It's probably best to buy one at a local gun show, so you can visually inspect before purchasing.
The RL ultralight stocks are the best profile of them all, IMHO.