Numbers A00 to E99 were temporarily used during a test 7/1967–1/1969. Cars could only operate in trains of the same letter groups. Cars E80 to E99 were not changed mechanically and served as a control group.
Source: ERA New York Division Bulletin, Vol. 49, No. 3, p. 10 / “A History of the R-1 to R-9 Passenger Car Fleet” by Eric Oszustowicz with assistance of Bill Zucker, Bernard Linder, Douglas Grotjahn, and David Ross.