Those are the later versions that I've reworked. Yes, those bosses were originally intended for attachment of the small heat shields to the undersides of the carbs, but I repurposed them for the bowl drains. Milled them flat (for a good sealing surface), drilled through, and tapped.
I don't think I've ever seen a pair of really early flat tops with my own two beady eyes, but it appears from the documentation that those bosses have been there since the beginning. Here's a snippet from one of the docs where they compare the old and new style float bowl covers. First (early) design on the left, and second (later) design on the right. The first design cover is flat on the outside, while the later design is not flat on the outside because they have the recessed pools for additional fuel. But they all have the heat shield mounting bosses:
And about the floats... The float design is very different between first and second designs and I don't think there's any possibility that they are interchangeable. And even if you find you CAN physically mount the wrong design into place, I don't think you want to do that. On the early design, the float mounting ears are on the inside surface of the bowl cover, while on the second design, the float pin goes through the body of the carb itself and is sealed with the brass hex cap. Completely different.
Here's a snippet from the docs that shows the differences between the two. New (second design) on the left, first (original design) on the right: