A 512Ke maybe or an educational model? I am sure there was a 512k model on sale for at least part of the Plus's lifespan. It wouldn't surprise me if the 800k drives from the Plus were used in it since it wouldn't have made sense to keep the 400k drives in production at that time. It also wouldn't have made sense to keep the old keyboard minus the keypad in production, either.
Edit
LEM shows this
http://www.lowendmac.com/compact/macintosh-512ke.html
The 512Ke did come with the 800k drive built in. It would likely have shipped with the full sized keyboard, too. They probably put the 512k/800 designation on the box so the two different models wouldn't be confused on the shelf. You wouldn't want to sell someone the older model with the 400k drive if they paid for the newer one.