The traditional two-letter phone number prefix was officially abandoned in the 1960s.
From time to time, as readers of this site know, they sometimes reveal themselves in faded ads and randomly found signs, like these two below.
I’m not exactly sure what an asbestos curtain was, but American Stage Equipment sold them from an
East Harlem office South Bronx office.
The CY exchange is new to me, but according to this guide, it stands for cypress, which places it in the Bronx. The sign hangs in an antiques shop in Brooklyn.
ST could have stood for stagg or sterling in Brooklyn, stillwell in Queens, or Stuyvesant in Manhattan. It was found on a Flatiron building, so Stuyvesant is a good bet.