Sometimes, the problems Apple had in the 1990s show up in support documents:
Keep in mind that there are two different versions of an Apple “Macintosh 6400” computer: a Macintosh Performa 6400/200 and a Power Macintosh 6400/200. These two computers differ in the bundled hardware and software configurations.
The Macintosh Performa 6400/200 has the modem port covered because it has an internal modem installed in the communications slot. When an internal modem is installed, the external modem port is not functional so Apple provides a port cover.
However, the Power Macintosh 6400/200 does NOT have an internal modem installed. Instead, this configuration has an Ethernet card installed in the communications slot. Having an Ethernet card installed, does NOT disable the modem port. So, if you have a Power Macintosh 6400/200, the modem port is still functional.
The confusion arises because some Power Macintosh 6400/200 computers have inadvertently shipped with the modem port covered. In these cases, if you remove the port cover, you can use the external modem port.
Perfect.