This is a groovemount adapter for the Mosquito hotend by Slice Engineering. I use it to allow direct-drive use with Bondtech BMG extruders. The hotend's included M2.5 mounting bolts point upward and screw into the captive nuts, which you press-fit into the hexagonal slots.
This is a work in progress--proper pictures will be coming soon!
Doesn't matter, but use at least three or four perimeters