The original MPU-401 compatible interfaces from Voyetra (nee Octave-Plateau)
The interfaces consisted of an 8-bit ISA card with an appropriate Connector
Box (it its 25 pin port).

The OP-4001 was manufactured 1983-1986.
(OP stands for Octave-Plateau, which was the previous name of Voyetra
Technologies.) The CB-4001 (Connector  Box) provided Midi In, Midi Out,
Metronome Out, FSK Out, FSK In, and a toggle switch for the FSK.

The V-4001 was manufactured 1986-1989.
The CB-4001 (Connector Box) provided Midi In, Midi Out, Smart FSK In & Out,
Metronome Out, and toggle switches. *> The Connector Boxes for the OP-4001
and V-4001 are different, as one is a female connector and the other a male
connector.

The V-4000 was manufactured 1990-1992.  Original MSRP $ 139.95;
The CB-4000 has only Midi In and Midi Out.