Employment agreements from before and after closing, payroll and tax records spanning the transaction, a continuity-of-service confirmation, and any letter recording the transfer of staff to the acquiring entity. Add organisational charts for both periods so duties can be read against dates.
Work out who kept which records
After an acquisition the evidence is usually split between systems. Deal counsel or the corporate secretary holds share transfers and resolutions; the acquired company's payroll provider may hold records the buyer never received; a departed manager may be the only person who can explain a numbering change. Identify a custodian for each item early, because retrieving files from a decommissioned system takes far longer than requesting a current letter.
Add one custodian to that list who is easy to overlook and often holds the decisive material: the acquired company's former external advisers. An accountant who prepared statutory accounts, a payroll bureau that ran the old system, a company secretary who maintained the register — each may hold records the buyer never received in the transaction, and each will respond to a request from the right person while ignoring one from a stranger. Establish early who can authorise those requests.
Hypothetical example: a fitness studio group's acquired company changed payroll providers at closing, and the only complete record of the applicant's employment for the two preceding years sits with a bureau that will release it to the former director but not to the buyer's HR team.