It has a distinctive appearance appearance with its plastic collar. These S400PX cells produce 1.55V which is different from the 1.35V produced by the original PX400 mercury cell. Because the S400PX silver oxide cell produces a steady 1.55V, you are able to compensate for the higher voltage by a fixed number of stops or by adjusting the ASA setting. There may be as many as two f-stops underexposure by a camera that uses 1.55v instead of the original 1.35v, but the compensation you require is constant. You may find alkaline equivalents that fit perfectly but, as with all alkaline cells the voltage is not stable, it drifts down from 1.5V towards 1.35V (the voltage of the original Mercury cell) and then lower still. As the voltage is not steady throughout the useful life of the cells, you must constantly adjust how you compensate.