With internal records: design documents, project histories, training materials and staffing data that show what the proprietary element is, who created it, and how few people in the group can work with it. Testimonial letters help only when they point to those records.
Use the company's own archive as the evidence
The most convincing L-1B files read like an internal history. Start with the artifact: a dated specification, patent filing, validation report or software repository log that shows the proprietary process exists and when it was developed. Add the applicant's contribution: authorship credits, commit records, project sign-offs or performance reviews naming the work.
Then supply staffing context: an internal memo stating how many employees hold comparable knowledge and what training a newcomer would need, with a sample training plan if one exists. Hypothetical example: a Lac-Beauport process engineer at a specialty-materials company is proposed for a U.S. transfer; the file includes the plant's batch-control procedure she wrote, the revision history showing her authorship, and a memo from the technical director confirming that only she and one retiring colleague can commission the process on new equipment.
Letters from managers are useful as a guide to the exhibits but carry little weight alone. Finish with the standard corporate and payroll records establishing the qualifying relationship and the year abroad.