I finally found the rarest component of the add-on options for the Wheelwriter 3, 5, and 6 -- the display option. My Wheelwriter 5 now as all of the options installed, including the LCD display, the printer option, and spellcheck.
These options were sold separately. It took me a while to figure out that card number 3, the printer card, sometimes has the parallel port and sometimes not. And it sometimes has the RJ45 jack for the display option and sometimes not. At last, on eBay, I came across a printer card that has both the parallel port and the display jack installed. I'm pretty sure it was new old stock. A complete kit for all the options requires three circuit boards.
There are other posts in this group about the other cards in the add-on options. A Wheelwriter with any of these options installed will have an extension on the back of the printer to house the circuit boards. The extension has the label "5441."
As for the display, I have not been to find IBM's documentation for how to use it. I'm not sure I've discovered all the control codes. But it seems it's a one-line device. You can type a line that appears only on the display and make corrections as you type. Then pressing the carriage return key will cause the Wheelwriter to print the line. If anyone knows where to find IBM's documentation for the display, please share.
Out of sight behind the Wheelwriter in the photo is a small Lenovo computer running Ubuntu Linux. The computer is connected to the Wheelwriter's parallel printer port.
For those of you who notice the modem, I'm experimenting with modems over VOIP lines. Sometimes a 9600 baud connection is entirely usable. There aren't many places to call these days, though, which is why I'm setting up a BBS system. Some modems can send and receive faxes. If anyone with fax capability is interested in starting a typewriter/fax network, please send me a private message.