The letter appears genuine. Dale O. Cox was an attorney in Cleveland. If you note in the letterhead of the original letter, Dale O. Cox is listed about half way down the right hand side of the stationary. That means he was a mid-level associate (not a partner) when the letter was written. This Dale O. Cox would have been aout the right age.
Supreme Court of Ohio / Public Attorney Information
Roetzel and Andres was a law firm in that era and still exists today.
Roetzel & Andress firm overview
There is a Cox at that law firm who would be about the right age to be Dale O. Cox's son.
I think this myth has been NOT busted.
And lest there be any doubts:
http://www.snopes.com/business/consumer/browns.asp