Most of the Okidatas from the period (Microline 320, 520, etc.)
had a standard Parallel port built in on the back of the printer.
Right next to it is usually a plastic knockout.
If you get the RS232 serial port option for this printer,
you remove the plastic knockout, and slide the RS232 board
into the opening. There is a card-edge connector that
it mates with.
You don't have to set any switches to use it.
I don't really know if it will let you do "dual porting" or not.
(i.e., 2 computers printing to it, one via serial, one via parallel.)
As for the RJ45 adapter; those are fairly common.
The serial port probably only needs 3 pins for printing,
Ground, TXD, and RXD . . (7, 2, and 3), although they
may have included handshaking signals. Who knows.
A standard serial cable to/from your computer should work,
as well as a parallel cable.
Documentation should be readily downloadable
from the Okidata website.
As info. . . .
T