Ah, there's the problem.
Someone somewhere back in history either replaced this or assembled one from a couple cannibalized other ones and just bolted it together instead of shaving it down to fit.
The force from recoil, dropping it on the ground, buttstroking somebody, or whatever, is supposed to hit where the back of the receiver meets the bottom of the "U" in the meaty part of the stock. The angles on the forward part of the stock "ears" meet up with the angles on the rear of the thick part of the receiver, and as you tighten down the main bolt, everything pinches together and locks up as one solid piece. With that gap there, the thin "ears" we're absorbing all the force.
It would've fit loose and sloppy to begin with and only got worse as things inevitably started cracking.
Countless stocks are made like this and it's a long proven fine thing provided shit it shaped and fitted right to begin with.
This person did not do that. They just bolted shit together then when it was floppy and eventually cracked they didn't look in the mirror to find the source of their problem but rather blamed the piece and tossed it in a corner to age and rust.
As the kids would say...Big Will can fix it!
Time to get to whittlin'.