EW-SD50 x EW-SD300 Electric Wire Adapter
It is about 31mm long and 8mm in width / diameter.
Use with the 12-speed Di2 and SM-PCE02
If you want to update the firmware on 12-speed Di2 components, you'll need to use the SM-PCE02 tool. This tool was released a couple of years ago, long before the EW-SD300 wiring standard even existed.
As a result, you need to use an EW-AD305 adapter to connect the SM-PCE02 to 12-speed bikes. They're not expensive, but it's something to be aware of.