This connection is specially designed for use on pipes as it has a screw to tighten the coupling to the pipe. Used, for example, when connecting drinks on beer tower with 8 mm (5/16") connecting pipes.
These couplings are completely interchangeable with couplings from affordable Fluidfit, these can be found under accessories.
Fluidfit HUCP Straight connection for metal pipes (inch)
Fluidfit HULP pushfit elbow connection for metal pipes (inch)
To find the right one, you can start from this table. Keep in mind that it can hit up to 5 mm when measuring over threads so it is not accurate but indicative.
R - Inch - Inv thread - Utv thread
R6 - 1/8" - 9mm - 10mm
R8 - 1/4" - 12mm - 13mm
R10 - 3/8" - 15mm - 17mm
R15 - 1/2" - 19mm - 21mm
R20 - 3/4" - 24mm - 26mm
R25 - 1" - 31mm - 33mm
R32 - 1 1/4" - 39mm - 42mm
R38 - 1 1/2" - 45mm - 48mm
R50 - 2" - 57mm - 59mm
R65 - 2 1/2" - 72mm - 75mm
R80 - 3" - 85mm - 88mm
R100 - 4" - 110mm - 113mm