The one pictured is a 14mm to 18mm male GonG, so it is both.18mm or 14mm?
https://www.ebay.com.au/itm/14mm-Male-to-18mm-Male-Glass-Adapter-Joint-Slide-Bowl-Extension-Hookah/302559014692?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
You may need bigger O-rings to make it fit snug as a bug
Or it could b too tight and will need a drill
https://www.ebay.com.au/itm/Diamond-Tool-Drill-Bit-Ceramic-Tile-Glass-Hole-Saw-7mm-3-11-2-pcs-D5Q3/183030982661?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
It is a real pain when no two fit the same!