That break looks exactly like what happened on my RBL 12ga. I was using reloads with Cheddite primers. I had noticed that these primers were prone to piercing but didn't think much of it since I never had any negative misshapes from previous pierced primers of any make. At the shot I heard a pop on the right side of my face and saw something fly. The pierce primer dumped enough gas into the box lock stock space to blow the check of the stock off. Certain guns are prone to piercing primers and certain primers are prone to piercing. When the two meet up things can happen. I am careful what I use Cheddites in now. After unsuccessfully trying to glue the check back on, I did a butt transplant. I preserved the nice piece of walnut CSMC put on the gun and have an improved checkering pattern to hide the seem.
