Well, you posted two letters below in a previous post. The writing in both of those are completely different. I would say if any of them are authentic only one would be. Hopefully someone here familiar with George Reeves will chime in. George Reeves was my Superman!!!!!( showing my age)
It's way off, but there is a possibility his mother wrote it. When she was visiting, she would often help him out by fulfilling autograph requests since he got so many.