Dear Anu Monu,
If the company did not mention the designation on the monthly payslip, then it shows how unprofessional they were. You could have left the company long ago.
Anyway, as suggested in the previous reply, visit your company. Talk to HR and also the MD of the company. Make an earnest plea for the issue of the "Career Certificate". In the certificate, they should mention designation-wise break up of your tenure with them.
By chance, if the company shows apathy or even hostility, remain calm. Do not lose your composure.
Before you visit your ex-company. Ask them to issue a letter to you to furnish details of the promotion or the "Career Certificate". A letter from the current company will lend weight to your request.
Thanks,
Dinesh Divekar