My mistake, I hadn't noticed the internal corner pieces, however they do appear to be removable, it also appears that as in my previous post on this subject that the external corner pieces can be moved and replaced as requiredWithout the little corner pieces, it would probably come in handy for making router table holes. My mom had kitchen cabinets with veneered particle board doors. They had a routed design like in the pic. The design was painted black and kinda tinted along its edges. They looked quite nice considering the material used. I wonder if that rig was for something like that.
View attachment 402049
I see where you're coming from on the internal corners, I'd also say that the external ones could give a different shape depending on the size of the router baseplate, from the photos shown it also appears that the outside corners can be adjusted, any basic rectangle/square template will give you the shape with rounded corners, but utilising the external corners shown would allow some adjustment in the corner radius, unless the external shapes are just there as reinforcement and for no other purpose, thanks TenGees your idea is food for thought, I'm off to work at the Edinburgh Fringe soon so probably will not be able to comment or view a lot of things on the forumIt seems to me that the bigger curves are just to allow for the baseplate of the router to get by. A bushing or bearing would ride on the interior of the adjustable rectangle but the baseplate would ride on the bars. The little corner inserts could make a shape like the drawing I posted. The thumbscrews in the large curves are the locks.