They look to be mother of pearl, but you need to tell us if 'you' think they are plastic . The medallions appear to be from the 1910 decade but the inletting job is suspect unless they have been damaged. My guess is the "diamond" is added material and at the time of the modification the escutcheons were switched to the opposite side, as the stock screw on these should start on the left panel.
They are not plastic. I wondered about the screw coming in from the right side stock. Wish I had an interesting story to go with them. Garage sale. I thought they were ivory, but now, you are correct .MOP.
Definitely not factory. The MOP material is substandard and would not have been used by the factory. The escutcheons and the screw are backwards. The medallions have been inserted too high in the grip panels. Appear to be home made, to me. Ed.