Would anyone know the gauge size I use to slip on the nipple end of a re-gasing point mine is 15.5mm but someone told me its a standard 1/2 european but

this is to small.  Links to a UK web site I would be greatful for this.  To give you some idea below is a link waht i'm after.  Many Thanks...

https://www.ebay.co.uk/itm/232554791084?mkevt=1&mkpid=0&emsid=e11051.m43.l1123&mkcid=7&ch=osgood&euid=a28c4aa296914260882c4e34497c90c9&bu=43203167511&osub=-1~1&crd=20220705010536&segname=11051

Low pressure and high pressure ports are different sizes to prevent mistakes.

Otherwise they're standardised so that thing will fit the correct port for filling and measuring.