A type of symmetrical variable area sound track recognizable by the fact that in the print the central axis and surrounding modulated area are transparent, whereas as in the duplex type they are opaque.
"I wanted to thank you for the wonderful work that you did on the 5 home-recorded 78 records that I sent you. These were made by my Grandfather in the late 1940's and 1950's. No one in the family had heard them in almost 50 years. They varied in what shape they were in, and we really thought that they would not be playable after so many years. ..."