My ZS carbs don’t have these brass plugs. The threaded opening on the flange is there, but it’s a blind passageway. I don’t think I’ve seen them before?
I also noticed the four screws on each of your carb tops - some ZS seem to have slotted screws while others have pozidriv. I’m not sure what’s correct? I thought there might be a correlation between slotted screws with one screw longer, and with a hole in it for the anti-tamper seal (later cars). And others with pozidriv screws for the carb tops that have the holes drilled through for the anti-tamper seal (earlier cars). However, I don’t think I was able to gather enough evidence to convince myself of a correlation. We talked about it a little bit in this thread: