I dont think thats accurate. I have seen many of the same keyboards on both the II and II plus. IF you speaking of the original (larger of the two datanetics keyboards) Datanetics keyboard (which is NOT the only keyboard used on the original II) yes its proprietary.EDIT: The Apple II Plus keyboard is different from the Apple II keyboard and does not contain 5740 keyboard encoder so I won't be able to troubleshoot by swapping this part.
Thank you for this information as the first thing I did was swap the entire keyboard assembly for my known working Apple II Plus keyboard. It worked better than the original keyboard but there were still issues. If they're interchangeable then this would suggest a problem on the motherboard. I'll try the reverse of swapping the Apple II keyboard into the Apple II Plus.The Keyboard encoder is different but you can swap in a complete keyboard with encoder. That’d tell you if the issue is on the motherboard or not