The MF-23 Multi-Control Data Back is the Big Beast databack accessory for  the Nikon F4 series cameras.  It was far more capable than the much less expensive MF-22 Data Back and relative few were sold.

For some reason, the MF-23 instruction manual is even harder to find.

This Nikon MF-23 Instruction Manual has a bit of wear, and minor tear (repaired) but is otherwise tight, flat and complete.

US shipping via Media Mail with corrugated and padded protection.