This B unit locomotive was built in March 1950 by EMD for ATSF as part of order 3020B. It was rebuilt in October 1973. ATSF ordered many of their F7 A and B units to be run in groups of four and had them numbered the same with letter designators to indicate their relative position when working together: “L” designated the ‘lead’ unit; “C” the cab, or second A unit, and A, B or C for the B units. The 231L unit likely started its work on the rails followed by A unit 231C and B units 231B and 231C.