The impact that an LOR's letterhead has will likely be minor compared to other factors such as the content and credibility of the recommender.
Since AMO was founded, we’ve helped 5,500+ international medical students and graduates rotate in the U.S. Many of our trainees have earned LORs from their AMO programs, and subsequently have gone on to successfully match into U.S. residency programs.
Many residency directors have told us they are using text recognition software to evaluate LoRs, as they are tasked with evaluating pools of thousands of applicants. In these new systems, letterhead is never even available to be seen.
In addition, multiple studies have been done to determine what the most important factors for a good LoR in the eyes of residency directors. Not one study concluded that the letterhead was a factor. Much more important was the quality of the letter, the qualifications of the letter writer (academic credentials), and any personal connections the writer may have to the evaluator.
Here are some sources you may refer to:
Program Fit and Communication Skills: RPDs place significant importance on the applicant's program fit and use the recommendation letters to assess communication and interpersonal skills, knowledge base, and experience [(Butts & Smith, 2015)]
Personal Connections and Research Experience: For orthopaedic trauma fellowship applicants, letters of recommendation, personal connections to the applicant and/or letter writers, and research experience are crucial [(Sandhu et al., 2020)]
Interviews and Interpersonal Skills: The interview and ability to work with a team are highly important, and recommendation letters from division chiefs and clinical faculty are deemed most valuable [(Poirier & Pruitt, 2003)]
Residency Program Reputation and Comparative Statements: Residency program reputation and the strength of comparative statements in recommendation letters, especially from residency program directors, are associated with higher evaluation scores in cardiology fellows [(Cullen et al., 2020)]