one is a joker type and the other a flapper. They are not efficient at all if mounted upside down, and not very efficient even sideways. The flapper hangs open enough to leak the prime, hard to get it going again especially as the valves age. Even when new, you will find the pump works much better upright as the valve close more efficiently on each stroke. I learned this by installing 4 of them on my boat, and reinstalling one. Henderson doesn't seem to mention it much in their literature, but Lavac (which uses one) is quite clear that it will not work well on its side and maybe not at all upside down.