If it's experience letter, you typically get it when you resign from a job. If you don't have it, you can raise a request now, and it'll be dated when they approve it and send it to you. It doesn't matter what date it shows, until the time you can ensure the authenticity of the letter (company letterhead, etc.)
For salary, show your pay slips, joining letter, promotion letters, etc. These should be whatever format your company provides it to you.
However, I'm confused about your question - you say salary was under taxable slab, but still you didn't get a form 16?! Whatever may be the case, you could still file your taxes and show zero tax. Those returns would also be acceptable proofs.
Thanks for the reply.
the employment period was Oct 2010 to November 2012. So three financial years fall under the period. I am applying on basis of this employment. I have form 16 for the recent year but that is under different category.
Regards