Buy one (0 on sale)
Initially sold for ¥2,800
The Yamaha ADP-1 allows you to use mark 1 cartridges (the Yamaha DX7 ones basically) in mark 2 slots (Yamaha DX7II-FD, DX7II-D, RX5, TX802, and so on). While voice data from the first generation of 6op synthesizers is compatible with the second generation, it doesn't work the other way around, hence the need for a different size and connector layout of the new cartridges (that also include new features).
It's a very simple accessory, made of plastic, that reroutes the data from one connector to the other. In recent years, prices for the ADP1 have skyrocketed: anybody selling one of these for more than 100$ is just a thief.