I have taken apart my TI-Keyboard from circa 2002, and have made a little Imgur album:

https://imgur.com/gallery/uhEAOtt

The case and PCB are significantly different that the other teardown that exists, even though they share the same part number.

https://imgur.com/a/llMQU7n

Notably, on my keyboard, there are no longer provisions for the second link port within the case mold, but there are still unpopulated pieces for a second port on the PCB. The serial number format is different as well. The PCB sports the work Silitek on the silkscreen, a division of Lite-On according to the Deskthority Wiki https://deskthority.net/wiki/Silitek. The main chip on the PCB seems to be a very similar part, EM78P451A0 vs EM78p451SA0S. This seems to be a low-power one-time-programmable general 8-bit microprocessor. http://www.datasheet-pdf.com/PDF/EM78P451-Datasheet-ELANMicroelectronics-546102

I have also mapped out the matrix of the keys formed by the membrane, with the plan to replace the membrane with a PCB to support mechanical key switches.

Really neat! I'm not entirely sure what this is in aid of, but it sure could be helpful to have this information on hand for some hypothetical future project. Good job!

Did we ever hypothesize on the potential purpose of the second link port/do you have any hypotheses?

My hypothesis is as a pass-through, or transparent pass-through. Such that the keystrokes are only sent to one of the calculators, but the two calculators (or other peripheral) can talk to eachother through the keyboard.

I'll do some probing around and report back.