This oddity is in Central Park just below the Fifth Ave stone wall at about 102nd St. How long this slab of stone has been used to cover this hole, and exactly what is beneath the stone is known to just a few, and group that does not include this author!
The slab seems like a left-over from a gravestone yard sale, but it serves it's purpose and still has two of the four bolts intact to keep it in it's place.
Although unusual today, before the era of iron manhole covers many wells, springs and sewers were covered with stone slabs to keep wayward souls (or is that soles?) safe.
This item is unique in my experience in NYC. There are a few more-refined stone covers in Brooklyn. There is also an even more makeshift hole cover about a 10 minutes walk from this one (see below.)