Reference letter. Some say recommendation letter.
I see what you mean. I was thinking of the letter a job seeker is using.
OK. 正宗中国特色的介绍信 is something English speaking countries don't use. So there's no corresponding word or phrase in English. You may want to say "authorization letter" or "proof of identity letter". If you do, you must explain in a few sentences what it means and why it's needed.