Most of these rifles that were built a while back are probably opened up on the bottom of the action because back then, that was the only way to get ai mags to work.
Now, if you have a bottom metal or chassis for your rifle that takes ai mags, you can feed your m70 with MDT poly-metal mags without having the action touched. These mags are just a touch slimmer, enough to fit through the magwell and work.