It's a top trim attachment for a Grizzly or similar miter trimmer:
Top-Trim Attachment for G1690 Miter Trimmer | Grizzly Industrial
earl
Top-Trim Attachment for G1690 Miter Trimmer | Grizzly Industrial
earl
I wouldn't have either--but I bought a used miter trimmer last spring, and that thing was in the box but not covered in the manual. So, I had to do a little searching, but at least I had a hunch it went with the miter trimmer. By itself, or in a pile--wouldn't have known where to start!!Never would have figured that one out!