Complex sentence: Initial codes were'ALF','ALX' and'MLX' but with the introduction of four letter codes in 1979 they became'VMBX' and then'VMBY'.
Split into simpler sentences: Initial codes were'ALF' and'ALX' with the similar'MLX' added and reclassed later. With the introduction of four letter codes in 1979 they became'VMBX' and then'VMBY'.