Pierre Cartier allegedly won the 5th Avenue mansion location by trading two strands of perfectly matched natural pearls with Mrs. Morton Plant. The jewelry is still incredibly persuasive, from such established favorites as the interlocking rings to the more recent additions such as the handcufflike Menotte bracelets. Subway: E, V to 5th Ave./53rd St. Other location: 828 Madison Ave., at E. 69th St., Upper East Side, PHONE: 212/472-6400; Subway: 6 to 68th St./Hunter Collge.