The Acorn Master cartridge breakout board by Simon Inns:
This project provides a simple breadboard adapter/breakout board for prototyping cartridges for the Acorn BBC Master and Acorn Electron 8-bit computers.
The design consists of two PCBs, the first plugs into the computer’s cartridge slot and the second is designed to plug along the edge of a standard 2.54mm pitch breadboard. The two boards are connected by a length of 50-way ribbon cable.
Project info at waitingforfriday.com and GitHub repository here.