This is apparently a very common location for splits on both P14 and Model 1917 stocks. Here you can see an arsenal repair in the same location on my P14 stock. The repair which is very old was made with 2 dowels driven in from the top with no type of glue or resin present. The repair is very strong perhaps much stronger than the wood ever was on it's own. I believe the damage happens to that spot from outside the rifle rather than inside as you surmised. A solid bump to a fairly weak area is all it takes. That is my theory with no proof to the contrary so who knows? Bill