It simply uses a DB-9 to HD-15 cable that came with the monitor. Although it sounds probably that this is not the case with the old monitor described in the first post, it did apparently happen ...

And if you were to go back a few more years into the past, you’d have found when you bought a mouse with a PS/2 connector fitted, it may well have come with an adapter for a 9-pin RS232 serial port.