You put your foot on it to push it into the earth; the reason for the arched opening and wide area of steel. I'd say it's for potatoes or some other underground vegetable.

I agree. It looks like a variation on a broadfork, but meant for narrow beds or VERY heavy soil. The two inner tines create a lever, good for breaking soil and lifting roots. It would be nice for harvesting large carrots or parsnips.