Oh, now I see the Mayfly protoshield: https://www.envirodiy.org/mayfly/hardware/accessories/. It happens to be out of stock at Amazon at the moment, but that’s OK, I don’t have an immediate need. It would work, but would also require adding/moving the Grove sockets to the protoshield if one still needed those.
I was implementing a TTL-based sensor using a software serial port, and initially it seemed like it would be easiest to connect to the Mayfly’s digital pins via the 20-pin header, but I realized that I could instead use one of the Grove ports and a Grove to screw terminal adapter, so that’s what I did. In the future, if I need access to additional/different Mayfly pins, I’ll either get a protoshield or make my own.