Lower engine gasket sets for Austin B-series engines almost always come with two gaskets for the oil pump. The smaller one is for the 3-main-bearing engines, while the larger one is for the 5-main-bearing engines. If you install the smaller gasket in the 5-main engine it will leave a small gap at the oil pickup port, in which case it will suck air and not pump oil.

If you look at the wrong gasket when it is placed on the block, it looks believable that it is correct. It fits the studs, and it goes around all the holes in the block. If you place the wrong 3-main bearing gasket on the 5-main bearing pump, it becomes apparent that it is incorrect because it does not go around all the holes on the pump.