For many years throughout his career, he spelled it Satchell, it is not a misspelling. Some time after he retired he adjusted it to the post career more common spelling Satchel. Here is one of my documents of his with this spelling from the early 1950's....This is the more rare and desired vintage version of his signature.
Any questions, please feel free to ask.
http://postimg.org/image/60rofvpz3/