Mystery Carb Part. What is it???

You can put them in wrong, I did once and it fell out holding the float open, flooded the carb and filled the whole outside of the intake manifold with gas.
While driving down the interstate.

I had the tools to fix it on the spot but had it towed, because the oil level was "extra full" and the gallon or so of gas laying on top of the manifold.

Lesson well learned.