Those inserts are an attempt to stop the forend wood from splitting down the middle. I see that they only sorta worked. I also note that your stock head is split. Remington doubles are wonderful guns but every single one I have ever seen has the stock head spreading laterally apart. The metal acts as a wedge to drive it apart. Modern epoxies can fix this and prevent it happening again. Good luck on your project. Very worthwhile gun to repair and use.