I generally make my own side plates just because pre-made side plates assume a particular rate of curvature in the lock/wrist region. I think that a rifle looks best when the aft end of the side plate is centered on the curved line that extends from the center of mass of the lock flats through the wrist and on to the butt. As I am laying a rifle out I draw a line that is centered on the wrist and extend that curve up to and across the lock flat. The end of the side plate, lock flat and beavertail all get centered on that line.
Many of the original rifles have side plates that are off-center, tilted, almost breaking through the edge of the lock flat, etc. I have pondered this too. It has led me to suspect, like Acer, that sometimes the old-time makers were assembling rifles with pre-made, perhaps even purchased components.
Best Regards,
John Cholin