I’m trying to find info on this because I’m looking to sell it.

I ran across the following. Are the corner pieces removable? It looks to have been designed to automatically square itself up no matter what size you make it.
1962 Albert Wing Product Tempe Arizona Router Template Jig Tool Cabinet Joinery 1962 Albert Wing Product Tempe Arizona Router Template Jig Tool Cabinet Joinery for sale online | eBay

I'm recalling some old time cabinets that had a groove cut in a shape like that. This would allow you to do that on any size drawer front, for example. Can't find anything else about this jig. If the corner pieces come out, or are replaceable with other shapes.

I found this pix. We need a real old-timer for this.

Reading through the patent gave me sore eyes, but it boils down to what I thought it might be. Here is the meat and potatoes of the patent application:

"This invention relates to a routing template and more particularly to a routing template for use in routing various design configurations in flat panels such as cabinet doors and drawer fronts or the like.

In custom cabinet work, it has been a problem to provide a simple routing template which may readily be securely engaged with flat panel or flat panels of various sizes for the purpose of routing various ornamental designs therein.

Heretofore, various templates have been made for routing designs in panels of a given size, but it has necessary to tool up templates on a custom basis for each given panel, and thus, it has been prohibitive for cabinet makers or other craftsmen to provide rounded ornamental in various panels of various sizes due to the cost of such custom work."

One of the features of this device is that it can be affixed to the panel so it doesn't move while routing the groove. You fit it to one corner of the workpiece, then slide the opposite side to the opposite corner, then pull the other sides so they fit the corners. It stays parallel no matter what size rectangle the piece is. As someone mentioned, the rounded corner pieces are so the router base fits. The small pieces in the corners seem to be fixed.

A very long time ago, far far away, I remember having or seeing a dresser with drawers with this kind of groove, in fact, the same shape. Looked for a picture of the result, but can't find one. It's a little like doing an inlay, in fact you could probably do that with the right bit.