In my article (published in the Winter 2010 Collector magazine), I briefly discussed your question.

To back up just a bit and provide some clarification, there are three "variations" of the Winder Muskets versus three "models". Why Winchester cataloged the third variation Winder Musket as a "Model 87" is a mystery that will likely remain unsolved. The one thing that is certain, is that it was the final iteration of the Winchester Single Shot (Model 1885).

Bert