You have two parts that clamp to digital callipers and enable a precise measurement of how much filament (1.75mm) is drawn into the extruder when you are calibrating it.
One part clamps to the end of the calliper and has a PTFE tube inserted into it. The other end of the tube goes to the extruder.
It does not matter if the extruder does not have a fitting to take the tube as long as the end of the tube buts up against the hole in filament clamp for instance and the filament can be drawn freely though the tube.
The other part clamps to the moving jaw of the calliper and has a clamp for the filament coming from the filament roll.
To use it just slacken off the filament clamp screw, close the jaws on the calliper, tighten the filament clamp screw and zero the calliper. Then run 100mm of filament through the extruder as per a normal extruder calibration.
The calliper gives you a direct reading of how much filament was actually drawn into the extruder.
Note: In the early stages of calibration it may be best to only run 50mm of filament as the calipers (well, the ones I have) will have a max travel of about 105mm with the parts clamped to them.
0.4mm Nozzle, 0.2mm Layer Height
150mm Digital Callipers
M3 screws x3 (12mm long - does not matter if they are longer)
M3 nuts x 3
Length of PTFE tube 2mm inside and 4mm outside diameter