The specification itself with its version history, records showing he authored and revised it, training logs for crews he certified, and project files where the method was applied. Keep confidential detail summarized rather than omitted.
Let the version history tell the story
A specialized-knowledge claim is only as strong as the paper behind it. The core exhibit is the specification manual, or at minimum its table of contents and a representative chapter, together with a version log that shows who changed what and when. If the senior builder's name appears as author on the first version and as reviser on later ones, the authorship claim is proven without narrative.
Add crew training records listing the sessions he ran and the builders he signed off, and two or three project files, such as a municipal trail network and a ski-area downhill course, showing the specification applied from design to inspection. The firm can protect genuinely confidential techniques by summarizing them and offering full documents on request. The affiliate contributes its state-park contract, the scope of work, and a letter explaining why the specification is required on that project.
Finally, include his employment records for the continuous year and the ownership documents connecting the two companies.