The initial Y stands for Yang Ming Shan Management Bureau, which covered Shihlin District back in the days when Chiang, Kai-Shek lived at his residence (Shihlin Guan Dee) till his death in 1975. In a way it's a bit like Washington, DC to the US. Up until mid to late 1970, the ID was issued at age 12. Those with Y initial should have lived in Shihlin, not necessarily Yang Ming Shan, when they got their ID cards. I've never met anyone with Y initial bragging about it.