 Mods 0 to 3 were retired by September 1966 and the Mod 4 version would remain in service until 1990s.It's not clear exactly why the W59 was replaced by the W56 after deployment but issues with "... one-point safety" and "performance under aged conditions" were cited in a 1987 Congressional report regarding the warhead.Chuck Hansen alleged that all weapons sharing the "Tsetse" nuclear primary design including the W59 suffered from a critical one-point safety issue and suffered premature tritium aging issues that needed to be corrected after entry into service.