Based on the size in the picture, for it to be an Ataulfo it would mast likely have to be a nubbin as that is way smaller than any Ataulfo I have ever seen (but nubbins usually have normally developed seeds, just smaller). If all of the fruit was of the same size, that would eliminate them being a nubbin, plus nubbins are usually not harvested and worth of marketing. I would begin by contacting the buyer for the market to find out what it is, or what they purchased it as and it origin (if they will disclose the info). Something is just not right with the look and size of the fruit.