Actually it's Apple's own inconstancies. Apple used a closed-Apple only on the Lisa keyboards (which I attribute to the evolution to a single Apple key leading to the final ADB inclusion). But you are correct that none of the Apple series computers have a "backspace" key. However, the Lisa had the backspace "arrow" only and the Lisa 2 keyboard had just the word Backspace. Not sure if there were differences with the XL as it is difficult to know which is which. http://www.kbdbabel.org/lisa/index.html
http://pix.blakespot.com/view/computers/misc/apple_keyboard/IMG_2524.JPG.html?g2_imageViewsIndex=1
My thought here is that the keyboard was designed following the introduction of the IIc in late 84 as that design has been around since before then. The designer wanted a keyboard like this:
http://www.applefritter.com/node/294 but Apple nixed the idea as consumers would reject it as looking too fragile. In the case of the IIGS keyboard they compromised on the thin boarder around it, then fattened it up even more for the SE/II. Many people argued for years that the Mac Plus keyboard was the best Apple ever made for typing. This then is a perfect hybrid of a full-sized "business" keyboard integrating the more substantial Lisa 2 keycaps and Snow-White styling to offer a more "substantial" extended external keyboard for the IIc. In which case it would not have an ADB port, but rather a IIc connector and would have had to go in through the mouse/game input port. That seems like a stretch, so I opt for the "more substantial" optional ADB IIGS keyboard or prototype. Only a product brochure would determine for sure if it was ever in production. If it was a third party offering it was definitely unauthorized given its similarities to Apple's own product, certainly not an unlikely possibility.