The significance of the unsleeved fore arm upraised with bottle in hand is basically ; Who else could the person be , other than Oswald?
The only other possibility was Lovelady , since he is the only other person out there on the front steps who has categorically stated in WC testimony of having bought coke from the 2nd floor machine just before he then went out to to the front steps
Imagine if that original negative of Altgens produced a clear print that showed not only the arm with bottle more clearly, but some portion of the white solid oval shaped Dr.Pepper label on bottle in that hand?