Very interesting. For a long time I have known about push-pull soundtracks (two optical tracks side by side, identical except for being 180 degrees out-of-phase with each other) but have never actually seen a sample. There are no such thing as a stereo variable-density optical soundtrack (apart from the experiments by Alan Blumlein in the UK (circa 1934) and Ray Allsop in Australia (1937). Hollywood studios stopped using variable-density soundtracks around 1958. Besides noise reduction, the other benefit of push-pull soundtracks was the cancelling of ever-harmonic distinction, something that plagued variable-density tracks if exposure and processing were not 100% correct. No doubt your rare sample is on the highly inflatable nitrate base. The edge markings between sprocket holes might provide a clue to the date.
Thanks for your comment, it's always interesting with older film technology history. This piece of film is from the fifties. Fortunately, this is a safety film. This is printed on the edge of the film. So it is safe to have this piece in my home.
