[the 2 letter - 5 number system was a national program, so this should apply to Las Vegas as well - ]

In December of 1930, New York City became the first locality in the United States to adopt the two-letter, five-number format; it remained alone in this respect until well after World War II, when other municipalities across the country began to follow suit. From the 1920s through the 1950s, most larger U.S. cities used the Bell System standard format of two letters which began the exchange name followed by four numbers, as in DUnkirk 0799. Prior to the mid-1950s, the number immediately following the name could never be a "0" or "1;" indeed, "0" was never pressed into service at all, except in the immediate Los Angeles area (the "BEnsonhurst 0" exchange mentioned in an episode of the popular TV sitcom The Honeymooners was fictitious).

In 1955, the Bell System attempted to standardize the process of naming exchanges by issuing a "recommended list" of names to be used for the various number combinations. In 1961, New York Telephone introduced "selected-letter" exchanges, in which the two letters did not mark the start of any particular name (example: FL 6-9970), and by 1965 all newly-connected phone numbers nationwide consisted of numerals only (Wichita Falls, Texas had been the first locality in the United States to implement the latter, having done so in 1958). Pre-existing numbers continued to be displayed the old way in many places well into the 1970s. A Chicago carpet retailer frequently advertised their number NAtional 2-9000 on WGN until the 1990s; not to mention, the number TYler 8-7100 for a Detroit construction company, as well as another Chicago business, a carpet cleaning service, using HUdson 3-2700.

[Jim McNally - from the TUxedo exchange in Detroit]