Murph.
I like talking about the Merwins.
My research consists of my personal observations and Art Phelps book
The 1st and 2nd models both had scoop flutes.
The 3rd model had standard flutes and top strap.
The 4th model had the ribbed barrel and sight as you describe it.
For at least 10 years I looked at every one I could
The number of 1st and 2nd models is pretty low (IMO) so they could easily have been produced in the fist couple of years (1877-1878)
The 3rd models could have been close behind.
We will never know for sure
Mike