Current Designation is not mentioned on the experience certificate

I was worked in a company for 15 years. i joined the company as asst. manager in 2007 and later got promoted as manager and DGM. i left the company recently. During the promotions, i didn't get any letter from the company but my salary was increased and i used to dealt with the management, staff and clients with the same promoted designations. Now the management is saying that since there were no promotional letters, they will issue the experience certificate with same designation as mentioned in the appointment letter issued during my joining i.e. Asst. Manager. is there any other way to get the experience certificate with my latest designation. Kindly sugggest

This is a matter for you to negotiate with your current employer. They are the only people that can solve this problem.

Secondly, failing company assistance with this, and it becomes necessary for you to find a way to verify your claim to the more senior positions, what documentary evidence do you have in your own personal files?

You should always keep examples of work, letters, documents created during your career - providing they do not infringe confidential and proprietary information about the company - to verify points such as this. Do you have letters written to you that specify your designation as Manager or DGM?

Are there people willing to provide a referee report who will vouch for you as having being promoted to those positions? Can you talk to HR about documents in your HR file that refer to the promotions and relevant salary increases?

This may be one time you need to think outside the box - and next time, make sure you keep copies of evidence that you may need in future.