The twelve channel scheme, in order to maintain some bandwidth and routing compatibility, was carried through to the short haul carriers, as well, as they started developing to eliminate voice band open wire trunk lines in the 1950s.
N-2 was similar in heterodyning scheme, but in discrete transistorized "plug-in unit" architecture, while N3 used the same frequency plan but a scheme of using single sideband with a different voice channel on each side of the carrier, a technique first seen on the 16 channel Type "O" open wire short haul carrier of the 1950s.
Type BN was used at times by Bell Operating Companies after the 1956 Hush-a-phone Decision of the US Supreme Court, a landmark case which challenged AT&T's "benign monopoly" of US telephone equipment industry.
Part of this settlement was for AT&T's Bell Operating Companies to buy and use small amounts of Lenkurt and Collins microwave and carrier systems.
In California, Type BN was used almost exclusively to provide trunk and private line connections between Pacific Tel.