This cartridge is part of the MONEGI SmartPack Development Kit.
The Monegi FlashWriter-N64 cartridge was apparently used for programming N64 Development Flash cartridges. This is essentially performing the same function as the NU64 Flash Gang Writer system.
Using the Monegi SmartPack dev kit and some proprietary software, developers could program a ROM image directly into the flash memory of an N64 development flash cartridge.
Images Provided by @GamingLegend64 (Twitter)
For those interested you can download the Japanese User Guide for this cartridge. (Link below)
Scanned and provided by @GamingLegend64 (Twitter)