just searched that part number on e bay, looks like a guy had one made and some extras to sell.
I did a "home made" conversions, I have 2 wired remotes, helm and transom. My main function I really wanted was volume and it works at the helm but for some reason not at the transom which seems odd as the "home made" conversion happens after the splice. Could very well be something with the transom remote or the wiring between the Y splice and remote.
Nice find though!!