I can't find this stamp anywhere and therefore can't age these plates

I have 8 of these Johnson Brothers salad plates. I can't find the pattern but I'm not so worried about that. I love them and intend to keep them. It's the crown less stamp that is the mystery. I've searched high and low and can't find one like it. It may not show in the photo but there is also a tiny grey x on the back too....barely visible.