Thanks to the Proquest Historic Newspaper Database, I found an ad in the Nov. 13, 1949 NY Times for the Soundmirror Tape Recorder selling for $199.50 with microphone that "records for 1/2 hour." It was being sold by G. Schirmer stores in NYC and Brooklyn.
Claude B. Zachary
University Archivist and Manuscripts Librarian
University of Southern California
Doheny Memorial Library 220
Los Angeles, CA 90089-0189