Sure looks like a metate and mano from here but as quito said, any sign of wear on either of the pieces. I have several metates that were flipped over. One of my best was found flipped over when metal detecting . I was laying over a 1860's military shirt button.
That caught my attention when you said you flipped it over. From what we could tell in SW Oklahoma, before the horse, they were made from local materials, and these were too heavy to carry along when they moved, so they would flip them over to protect the grinding surface, and have them on site ready for use when they returned, if they ever did.