With so many models being built on the exact same frame (25,27,28,29,57 etc) it is understandable that from time to time the wrong model number gets stamped. It is a fairly common occurrence. Or as Doc44 stated, they may have intentionally grabbed a 29 stamped frame and built a 57 out of it because they needed a 57 frame at that moment and that was what was at hand. Sometimes guns are converted to other models by swapping barrels and cylinders. Any of the above is a possibility with your gun. But the overwhelming odds are it is simply stamped with the wrong model number. When you hear hoof beats, look for horses, not zebras.
Some sellers will try to get a premium for such a gun calling it a rare factory mistake, but in fact it's not all that rare and most people will not pay extra for one. Or less, either.