If you look closer at these carbs you will notice a couple of irregularities. The vent hole in the facia of the carb is not there and the auxiliary venturies are extended beyond the air horn.
This set up was only used for the 1725 cc Rootes Hillman Holbay motor. The float chamber breathes through a spacer with holes under the jet cover. That was because for some reason the normal breathing could not function with the original air filter housing. The extended aux venturies were to combat a flat spot.
Obviously pretty rare, just because of small production numbers of the car.