Historic share registers, acquisition or incorporation documents, group financial statements from the relevant years, and any filings showing the entities were related at that time.
Historic evidence, not a current organisation chart
A present-day structure diagram proves the position today and nothing about six years ago, yet it is what companies usually supply. What is needed is dated evidence. Certified share registers showing holdings during the relevant years, the share purchase or subscription agreements by which each entity entered the group, articles of incorporation with dates, consolidated group financial statements naming the subsidiaries in the years in question, and any regulatory or tax filings that identify the corporate family.
Where records sit with a foreign affiliate, allow time to obtain them; overseas company registries and former advisers are not quick. Then produce a one-page chronology stating, for each year of the employee's service, which entity employed them and how that entity was related to the petitioner. That single page does more than a folder of undated documents.