Did you measure from the end of the barrel all the way to the face of the cylinder? I'm not trying to be contentious, but it really looks like a 6" to me.
For reference, this is my 6" pre-27:
And this is my 5" 27-2:
If you compare the distance from the end of the lug to the end of the barrel in these photos (or the end of the Smith & Wesson lettering) to your photo, yours looks a lot more like my 6". That being said, it would be far from the first time I misidentified something from a photo!
Either way, sweet revolver!