The PLATE, seating was a means of saving L42 fore-ends that had been previously crushed under the screw. By milling the area out and inserting the plate, it effectively spread the load imparted by the trigger guard and screw over a greater area. It was one of those ideas that was good in both practice and theory so in the interests of standardisation it was adopted across the board in all subsequent fore-ends. It follows that not all L42's will have the seating plate. But you can't have a seating plate AND collar. One or the other..........
I'm not sure that it was adopted across the board for the L39
The seating plate was not a new idea. Fultons p[atented the idea before the war as I understand it and although the other rifle specialists wanted to use the idea, they couldn't. Enfield/the Government didn't worry about such trivialities as property rights and patents and told Webley and Sterling as much. So they just copied the idea anyway. But they were concerned when the patents and property rights belonged to them and told Lithgowas much when they wished to convert their own L4 guns................. But you get my drift!