If the stone carver was just told "do a man in a fish costume holding a bag", the bag would look different each time. But it doesn't, it's identical across the continents and cultures. It occurs to me that, for them to look exactly the same wherever the stone reliefs are found, they must have still existed at the time the images were carved. The stone carver would be familiar with exactly what they looked like.
I wonder if any are still in existence today? Is there one sat in a small museum somewhere, and they don't know exactly what it is that they've got?