The company's own procedure specifications and qualification records naming him, training records showing he certifies others, nonconformance and inspection records relying on his judgement, and a written comparison against the public welding codes showing what the company added.
Separate the code from the company's procedure
Every certified welder works to published codes, and an adjudicator will presume that a competent U.S. welder knows them. The specialized-knowledge claim rests on what sits above the code: the company's procedure specifications for its overlay process, the parameters it developed, the fixtures and sequences it uses, and the quality controls it applies.
The documents that prove it are internal. Procedure qualification records that name him as the welder who qualified each procedure show origin. Training logs and welder performance qualifications he signed show that he transmits the knowledge.
Nonconformance reports and customer inspection correspondence that turn on his assessment show that the company relies on it. A short technical statement from the plant's engineering manager comparing the proprietary procedure with the base code, and estimating the training time for a new hire, ties it together. Redact commercially sensitive parameters rather than omitting the documents, and explain each exhibit in the petition letter.