Yes it was the SRE.
I thought I would add the number produced because of the many types made over the years.
About the best Waco coding information can be found at the aerofiles.com site. Just as they state: "
Ever a subject of contention, confusion, speculation, and bar-room brawls, Waco's system of codifying its planes gets somewhat clarified courtesy of Jack Wilhelm, of the Waco Historical Society, Ray Brandly, author of the definitive Waco book, and Aerofiles' own Lennart Johnsson."
http://aerofiles.com/wacodata.html
You have control...