I posted pictures of my rearsenaled P14 and M1917 in the past. In just the two examples I have they interchange and fit fine. It’s my understanding that the Brits did not have replacement stocks for P14s that were rearsenaled in preperation for WW2. M1917 replacement stocks were procured for that purpose. If any alterations were made that’s unknown to me. The original whale belly P14 stock was an odd duck.