Anyone know what this is? The flat piece passes thru a slot on the left side.

MMH has it right. My mother had one, made by Foley in stainless steel. You scoop and scrap the dough. If you were making cookies that you had rolled and chilled, you could also use it to slice and transfer the dough. Well, not large diameter rolls- but the tiny ones made for ladies group get-togethers. 50's - 60's common kitchen gadget.

My mother had 1. She used it to make cookies. She scooped like a spatula and then scraped the dough on to the cookie sheet.

Not sure. Guessing some kind of fruit or vegetable pitter. If you can find a brand name and/or country of manufacture anywhere on this tool, it might help us figure it out.