Initially sold for ¥9,800
The Yamaha UDC01 is a static RAM cartridge that can hold data from various other Yamaha programs, such as YRM103, YRM105, YRM301, YRM302, YRM303, YRM304, YRM305, YRM506. A battery inside the cartridge keeps the 4KB memory alive. It's effectively identical to the Sony HBI55.
Note that on MSX computers with a single cartridge slot it can't be used together with another cartridge unless connected to the back expansion port with an adapter such as the Yamaha CA01.