Check the pic I took. It looks to me like the 'nub" corresponds to the location of the enrichment (choke) rail screws on #2 & 4. All the carbs have the "nub," but only 4 of them are drilled and tapped and shaped for the choke rail mounting. It is akin to the loop on #4 that matches the loop on #3 where the idle adjust knob goes. I guess it saved manufacturing costs.
As far as I can tell, there are no passageways under where it broke off.
If it was a customers carb I would replace it because I don't want to send someone something I broke. If it was my carb, I would inspect it and verify no function is compromised. I would then put it back together and run it.
You can JB Weld the piece back on, but it probably is not visible with the carb rail covers installed, especially when the carbs are on the bike. Actually if you are going to break one of these off, #6 or #1 are the best choices!
Dave
