If I understand you correctly, I think you're refering to the NWMP carbine, not an 1876 Winchester Musket. The original 1876 model NWMP carbines were equiped with Spanish Meter rear sights. I can't speak for the reproduction, but I do have an original Mountie carbine with this sight and it is not adjustable for windage. I would not think that the Spanish Meter sight on the Chaparral carbine would be windage adjustable, either.
Adios,
Buck