The 721 was a billy club. It was heavy, the stock was poorly made, the action was smooth, but the extractor clip was it's downfall.
There was no half safety to unload the gun on the original design.

There is not too much of a difference between a 1955 - model 721 and a 1962 Model 700 - except for the fact that the stocks got better, the extractor was changed to a design which is shared with the 760 Gamemaster and there is a slight difference between the bolts.
I can't remember for sure, but I think that the locking lugs were made bigger for the model 700. A couple of minutes of machine work and the new bolt will work in the older rifle.

The barrel is definitely lighter in the model 700...

Plus the 721 never came with a factory trap door of clip which would have made it easier to unload then to take the bullets out one at a time with the bolt.