Ok, I do not think there is a fix pattern. What I have learned from others is that your exp letter should contain following information:
It should be written on a company letterhead bearing applicant’s name, the company’s name and contact information such as address, phone number and email address.
Should mention all positions held by you, stating your job title, duties and responsibilities, dates worked for the company, number of work hours per week and annual salary plus benefits.
Should mention full name, title and signature of your immediate supervisor and/or manager.