This is an adapter for the Python to connect to a smaller airator on your faucet.  The thread size is 13/16"-27m to ¾" GHTM.  An 18 or 19 x 3mm O ring fits in the top but it will work just fine without an O ring.  If sizing an O ring, bring fitted print to your hardwar store for presise fit.  This replaces the stock brass fitting that comes with the Python water exchange kit.  I printed this in petg but pla should work just fine.  This fitting worked on our first use of the Python.  I searched the hardware store for a fitting and nothing worked so I designed this rather than waiting to have one shiped from Python.