It looks aged, but I don't feel comfortable with this and would say it is not authentic. Ptolemy many different angles going on with. The inscription and signature is not all in one angle to the writing as you would expect.
On the fence about this, leaning toward forgery. Here is an authentic Buffalo Bill letter I obtained a while back for comparison. His handwriting was much more "staccato", and quite difficult to read, perhaps due to his advanced age. (For example, "Mr. Campbell writes he has engaged all the people we have room for on our cars...")
I´m not shure thats it´s genuine. The most signatures I´ve seen, the "C" in Cody have ended in a form of "e", but not raised as in your example, it seems tilted backwards and then it goes in a straight line towards and connected with the "o" in Cody. About the "o" I can say, that I´ve never seen and "o" look like an "o", it allways seems to be like a regular "v".
Last, as already mentioned, Buffalo Bill are normaly written with the letters in the same angel.
Could the W Cody be real and the Buffalo Bill be added? I've seen some where the person receiving the autograph would add the name to it to clarify if it was not legible.