KB,
you are quite correct. The 319,320 and 321 are variants of the 320. As such to operate on them you are required to have "differences" training. So SH at LHR operate 3 types: 75,76 and 320 plus 320 variants. As I understand it CC at LGw operate 3 types: 777;747 and 737. (within the 737 family there are several variants 300;400;500)