"The earliest [number] cards began with the candlestick telephones and fit on top of the transmitter. Early cards showed the exchange (office code) for the area being called by using a word and a number (station number) for the particular home. 'Party lines' were used when a few houses shared a telephone line. To distinguish which house a call was intended for, a letter was added to the number. The standard party line letters used by the Bell System were J, M, R and W." [see party line cards].
"In 1921, the cities of New York, Boston, Philadelphia and Chicago were designated for dial conversion using seven digits in total with three letters and four numbers. Other cities and areas at that time used only three-to-six digit numbering plans. The earliest use of two letters and five numbers [see cards] was in Los Angeles in the lat 1920s where there was a mix of six and seven digit numbers until approximately 1958, at which time seven digits became standard."
"New York City converted in December 1930 to using two letters and five numbers. Philadelphia converted in July 1946, and Chicago and Boston in 1948. Soon after WWII, all large American cities began the change to the two-letter, five number system."