Contemporaneous records: internal specifications with the transferee named, development and validation reports, revision histories, training materials the transferee wrote and problem reports the transferee resolved.
Contemporary records beat a letter written for the petition
Hypothetical example: A support letter is necessary but it is advocacy. What corroborates it is paper created for a business reason at the time. The internal process specification carrying the chemist's name as author, dated revisions showing tolerances tightened after a failed season, validation reports from a trial line, deviation investigations signed off by the chemist, and the training deck used to instruct plant operators all show the same thing from different angles.
Add customer complaint files that the chemist resolved where the resolution required knowledge of the company's own chemistry. Redact commercially sensitive details rather than omitting the documents. Alongside this, payroll and employment records must establish one continuous year of qualifying employment abroad within the three years before the petition.
Knowledge evidence and employment evidence are separate requirements, and a strong showing on one does not compensate for a gap in the other.