I recently bought a Beretta 390 Sporting for my wife. The stock had already been cut for a shorter length of pull, the comb appears to slide out of a grove in the top of the stock. I was told that this is a factory stock and that the combs pieces were interchangeable for adjusting the height of the comb. I have tried to find out some more information on this but so far no luck. Does anyone have any other information on these stocks?