1. NOC code should not be dependent on you Designation but on your roles and responsibilities. Search the NOC site and check what matches with the job you are doing in all the organizations. As per my knowledge - You can choose multiple NOCs if your roles and responsibilities changed in the organization over the time - I will suggest you check this forum for recommendations from experts in this regards.
2 For TCS ex-employees there is something called skill certificate. I think FDFU team issues the same. You can get the exact email by searching skill certificate from TCS in this forum. Once you email them, I think, they will ask you to list your roles and responsibilities and then they will forward it your last supervisor or supervisors to approve those roles and responsibilities. For exact process do an extensive search on this forum.
@ruchitajj
What NOC code did u finally end up using?? I'm in a similar situation and had worked with TCS, where the title is Asst. Systems Engineer, hence designationwise, it matches NOC 2173,but the roles and responsibilites match with NOC 2174!