I've never heard of one so my first thought would be someone cut it back. You might be able to tell from the barrel markings, although 5" still allows them to be fairly well centered. The other tell-tale would be how the front ramp is attached. I've seen cut down guns where the top strap grooves were visible under the front ramp as viewed from the muzzle. My gun smith buddy does it the right way by milling off the grooves, then pinning the ramp to the barrel and the sight to the ramp.
For what it's worth, since I think 5" is the ideal barrel length for the N-frame revolvers, a 5" M57 would be more desirable to me than a 6" or 4". My $.02 worth.
Dave