For 5 years I wore shirts with "FIRST Senior Mentor" embroidered on them and signed my emails with the title.
My "customers" (the mentors, students, and event volunteers especially in Maryland) usually had a good idea of what the role meant when I was wearing the shirt. They tended to get a lot of communication from me.
A little historical perspective. The official title "FIRST Senior Mentor" was established as part of a new generation grant funded by the Corporation for National and Community Service in 2004. I believe the title was dictated by the target recruitment audience of the original project. There was a generous mix of younger and more "senior" in the original class. I always found it an interesting play on words.
http://www.chiefdelphi.com/media/photos/16218 to see a photo of the original group.
Over the years, I also encountered quite a few mentors who signed their correspondence as "Senior Mentor" or wore shirts with the title, as reflected by their role on the team, from all the FIRST programs. In conversation, I would say "make sure you read the job description on the FIRST website in case you are interested in the future with being a Senior Mentor for all the teams, not just your team."