A honking musicological issue

George Gershwin famously incorporated four taxi horns into “An American in Paris.” The horns, which the composer selected in Paris and brought home to New York for the premiere, are identified in the composer’s manuscript score as
“A,” “B,” “C” and “D.”

Are those pitch notations, or just Gershwin’s version of
1-2-3-4?

The New York Times’
Michael Cooper explores this musicological issue, which is not as trivial as it may seem at first honk. Gershwin’s taxi horns, as heard in the Victor recording of “An American in Paris” made shortly after the first performance, are pitched A flat, B flat, high D and low A, producing tone colors and dissonances that are markedly different from the
A, B, C, D tuning heard in today’s performances:

http://www.nytimes.com/2016/03/02/theater/have-we-been-playing-gershwin-wrong-for-70-years.html

Michael Strunsky, a nephew of Ira Gershwin, tells Cooper that the original taxi horns have been lost.