This is the earliest (and rare) version of New York City's red "STOP" sign. It dates back to the late 1950s, and it replaced the previous octagonal sign, which was yellow in appearance and introduced by the city's then Department of Traffic in 1950.
Two mounting holes were used, and both sides of the sign were red. A second version would appear before 1960 as well, and six mounting holes were in use on that kind. In addition, the side that read "STOP" was red, while the backside was bare aluminum.