I do not find anything wrong in what your senior had said. What is wrong in the designation "Trainee - Jr. Officer"?
Anyway now there is no point in going back. I will suggest you on how to strike balance between your and your senior's expectations. You may phrase the wording like this:
With reference to your interview you had with us, we are pleased to inform you that you have been appointed as a “Jr Officer” in our organization with effect from XX/XX/2013. You will be under training for a period of six months from XX/XX/2013 to XX/XX/2014. Your designation during your training period will be "Trainee - Jr Officer". Once you complete your training successfully, your designation will be "Jr Officer" and you will be put on probation for six months.
Just check whether the above working is acceptable to you.
Dinesh V Divekar
17th September 2013 From India, Bangalore