This unusual metal gadget was designed to make stubborn jar lids easier to remove.
The adjustable mechanism tightened around the lid, while the serrated gripping surface helped prevent slipping. Once secured, the user could apply extra leverage to twist the lid loose without relying entirely on hand strength.
Vintage versions of these tools were often called jar wrenches or adjustable jar openers and were especially useful for tightly sealed jars and bottles.
Answer: It is an old-fashioned adjustable jar opener, also known as a jar wrench.
