The design and test record with authorship identified, internal procedures and work instructions, training material, any patent or trade-secret registers, and payroll records that establish continuous employment.
Keep the record as you build, not when you file
Hypothetical example: the company has the drawings but the test results live in a departed employee's notebook and nobody recorded who did what. That gap cannot be filled convincingly afterwards. From now on, keep design revisions with authors and dates, test reports signed by whoever ran them, failure analyses with conclusions, and work instructions that identify who wrote them.
Keep internal training records showing who taught whom. Maintain a register of what the company treats as confidential. On the corporate side keep share registers current and payroll accessible for at least four years.
Keeping this record makes future evidence assembly more direct; without it, the company may be unable to document authorship and development history reliably.