This package contains 4 PC4-01 NPT Straight-Thru Bowden Fittings.
NPT stands for National Pipe Thread and is an American standard thread.
01 threading is 1/8 inch diameter, similar to M10 but with a slight taper.
These fittings have a 4.1mm rear opening to allow the tubing to pass all the way through.
0.9mm pitch.
For 1.75mm filaments.