This is a router base attachment for the PFBS 12 A1 Parkside Multi-Grinder Rotary Tool from Lidl.
It's held fixed in place under the existing head collar nut.
The cut depth is adjusted by moving the bit up or down in the tool's chuck.
A 2nd option is included with dual access on each side of the base which could be useful in some circumstances.
Recommended to print upside down (base up) in PLA at 100% infill with 0.2mm layer height.