It stands for Original Gangsta, of course.
But seriously, my impression reading the article was that it was just a set of randomly chosen initials.
Oliver Goodman, or what have you.
original gangsta.. loll
You are correct, it is the initials of the person writing. This might be due to many reasons, anonymity if the letter was read by someone else (spys, war, etc) and of course the reader would know who the initials belonged to..