The employment agreement and its start date, payroll records and tax documentation from that point forward, and organisational charts showing the reporting line. For the earlier phase, invoices, service agreements and any intermediary company records show what the arrangement actually was rather than what it resembled day to day.
Retrieve the earlier records anyway
Even where the contracting period is not relied on, its records explain a history that would otherwise look like a sudden appearance on the payroll. Where an intermediary company held those records and the relationship has ended, retrieval takes time and may not fully succeed. Begin it early and note honestly what could not be obtained rather than filling the gap with a description.
Request the earlier records at the same time as the later ones even where nobody expects to rely on them, because retrieval from a former intermediary is the item most likely to fail and the one that takes longest to fail. Where the relationship has ended, ask the organisation whether its own accounts payable records hold the invoices, since a payer usually retains what a payee has discarded. If material genuinely cannot be obtained, record what was requested, from whom and when, and note the gap honestly rather than filling it with a description of what the documents would have shown.