The mixture screw is very probably behind this metal plug then...
I've heard of these hidden mixture screws before. Some manufacturers think they can setup their carbs The Only One Right Way For Everybody (tm).
Kymco also seems to fall into this category now ?
It should be possible to carefully (not to damage the mixture screw) drill a small hole in the plug and put a screw in it, which you can grab with pliers to pull the plug out... Just search the interweb when in doubt, I've seen some guides about this somewhere...