Maybe the changed the naming but it used to be that 1540 = 1510 base with the 1530 extension when you buy them together. Shown in the middle of this picture.
As Ic-racer told, there is no 1540 extension, a 1540 is a 1510 + 1530 labeled, and sold, together. Being a modular system, the only difference – if you separate the extension from the base tank– is the label in the outside stating the combined capacities of tank and extension.