It is likely that the numbers on your buttplate tang are engraved and not stamped. If so, it would appear you have a Georgia-purchased Enfield rifle musket acquired during the Civil War. The letter "G" on the right side of the buttstock seems to remove any doubt, presuming of course that the numbers engraved on the tang and letter impressed on the stock are original and authentic.
Some Georgia-purchased Enfields were given to Confederate forces from other southern states by Confederate ordnance officers during the war, much to the chagrin of Georgia Governor Brown!
There were eight Birmingham gunmakers with the last name of Millward, two of whom having the first name of William. It is possible that they were father and son, as city directories list Wm. Millward & Son from the 1840s through the 1860s.