Write a question about the following article: Labor unions generally ignored government employees because they were controlled mostly by the patronage system used by the political parties before the arrival of civil service. Post Office workers did form unions. The National Association of Letter Carriers started in 1889 and grew quickly. By the mid-1960s it had 175,000 members in 6,400 local branches. Several competing organizations of postal clerks emerged starting in the 1890s. Merger discussions dragged on for years, until finally the NFPOC, UNMAPOC and others merged in 1961 as the United Federation of Postal Clerks.  Another round of mergers in 1971 produced the American Postal Workers Union .  In 2012 the APWU had 330,000 members.  The various postal unions did not engage in strikes.

Question about the article:
How many years after the National Association of Letter Carriers began did it finally merge into the United Federation of Postal Clerks?