John Guest 12mm stem fitting to 1/2″ bspt male thread adapter. 1/2″ bspt is around 20mm across.
Use with 12mm John Guest and other brands that have a 12mm OD diameter tubing.
Ideal to use with the 12mm tee fitting to branch off to a different size fitting.
Most caravans and RVs use 12mm tubing on their water supply lines and the John Guest Speed Fit fittings.