This is a massage roller with 60 mm diameter including the spikes to stimulate the muscle and help with circulation and veins.

For assembly you need the printed parts and one screw (ZYLINDERSCHRAUBEN DIN 84 M6X75 EDELSTAHL A2 KP) and one nut.

To counter the screw, you have to heat the nut and then press it into the handle.

EDIT:
After all these years, I can summarise that the massager is very effective and structurally strong.