This is an M4 metric T Slot nut, that has to be used in slotted (6mm) Aluminum profiles.
I use them for the frame of my new delta printer that is in construction
The profile shown in photos is 20x40mm
printed with support in ABS
I print them in multiple copy at one time (of course)
Just remove by hands printing support and slightly smooth surfaces with a file
Press by hand an M4 nut in the receptacles. It will stay in position due to the low clearance between printed part and the nut