I was always under the impression that Chevrolet produced 453 SS models in 61'. Of that 453 142 of those cars were 409 cars but, I was just reading in Chevy Super Sports 1961-1976 By Terry V. Boyce and he states.
"Production of Super Sport kits for 1961 was very limited. Chevrolet indicates 453 cars left the factory with SS equipment. (Another figure quoted, of 142 cars, is apparently the number of 409-equipped 1961 Chevrolet's built.) There are indications that some dealer installations of the SS trim items were made very late in the year."
It appears that the 142 409 cars could have been Biscayne, Bel-Air and Impala. What has me a little puzzled is would all of those cars be equipped with the SS trim. It sounds like maybe not.