You are an observant one Sil! The reason is that the part number is the same, but the screw holes on the body are "clocked" differently as the carb bodies are mirror imaged. The little buttress was used for a passageway (small hole) drilled to vent the spindle bore when a "dustproof" damper cap was used. You'll see a small hole in the plastic cap on yours - earlier carbs had a solid cap so the spindle vented into the upper part of the chamber. And the buttress isn't drilled on those chambers.
After rebuilding a couple thousand of these carbs I figured it all out....
Rhys