I do apologize that no one has yet come along to address your question! In case you haven't done so, I would suggest writing such a letter like the following. It doesn't have to be long. It's just a simple request.
I'm also assuming that this is for your work history with the company and not concerning a letter of recommendation. If I'm incorrect about that, please let me know.
Dear [Name of the Person to Whom You're Making the Request.}
Please accept this correspondence as a request for documentation concerning the dates of employment for [Name of Company], job duties that I performed and responsibilities that I had.
I would appreciate it if I could receive this documentation at your earliest possible convenience.
[Your Typed Name.]
In the header of your letter you would need to have your name, address and phone number as well as maybe even your email address.
After that you would put the name of the person to whom this is being addressed as well as their address.
The date could come next.
For samples as to how to format a business letter, please click on the following link. Standard Business Letter Formatting - Google Search
I wish for you only the best!