Records created before anyone thought about a petition carry the most weight. Internal training curricula, versioned process documents with his name on the revisions, incident reports he resolved, and confidentiality agreements covering the material all show knowledge that grew inside the company rather than being asserted afterwards.
Contemporaneous records beat a well-written letter
A support letter is necessary and is rarely sufficient on its own. What makes it credible is the material behind it. For a process specialist that includes the document control system showing which revisions he authored and when, the training he delivered to colleagues, the commissioning reports from installations he stabilised, and the internal decisions that changed a recipe after something went wrong.
Where the knowledge is genuinely confidential, the agreements protecting it and the access controls around it are themselves evidence of its character. Contrast material helps as well: a job posting for an ordinary process technician at the same company, showing what is expected of someone without this background, makes the distinction concrete. Everything should be dated and consistent with the employment history, because the qualifying year is proved from the same records.
A file assembled entirely in the month before filing invites the question of what existed before it.