Early proto-NW guns used a flat sheet brass side plate. This was replaced by a cast serpent side plate by around 1745. This became standard (with some variation) through production of NW guns into the early 20th century.
Also... There was a unique style of trade gun traded to south east native groups during the later part of the 18th century that continued to use the flat plate. It was not a NW trade gun, but a separate style specifically used for trade in that region. Some today refer to it as a "Carolina Gun". Pieces show up in dig sites from the Mississippi river and south of the Ohio.