Another factor that seems to play into this is where these Macs were sold. The Mac ED, in any form, seems far more common in western Europe than anywhere else. European Macs often had different nomenclature in the early days (such as the 512K/800 and SE 1/20, the latter of which was just one of three SE configurations badged as such). Could it be Apple simply branded all Pluses sold to the education market as "Plus ED", similar to how all Performa 550s sold to schools were marked as an LC 550?
It seems many of the specification databases out there focus exclusively on the North American models. You won't find anything about the 512K/800 on most of the major sites even though the 512Ke went by that name in Europe. Also, I seem to remember the LCII being available longer in another country (I think it may have been Australia, but I'm not positive--if anyone could confirm this it would be great).
A good project for our wiki would be to list all regional variants and differences in configuration, date, and price.
A theory on the platinum 512Ke--with the Plus becoming the new low end, there probably wasn't a ton of demand for this entry level product, but there was enough to keep the computer around. Apple was moving towards high-priced Macs when the SE and II came out and wouldn't focus on the budget segment again until 1990. The boxes may not have been changed since Apple probably wasn't going to keep the 512K around for much longer after the new Macs came out. I'd be interested to see if any early 1987 Pluses, IIes, or ImageWriter IIs (all three of which changed color and, in the case of the IIe, keyboard design) came in old-style boxes designed for the beige versions.
Apple packaging can be very inconsistent. My Mac LC came in a brown cardboard box, while its monitor, an Apple 12" RGB, came in the older white box. The LC box must have just changed over, but the LC itself was discontinued a few months after mine was produced (mine was made in late November 1991; my monitor's date was October 1991).
In any case, I doubt Apple sold many platinum 512Kes. They didn't advertise it, as we've pointed out, but Apple sometimes doesn't emphasize an older product. The IIe and ImageWriter II were around until 1993 and 1996, respectively, but a lot of people probably had no idea they were still being produced at the end of their life cycles. More recently, the iMac G3 was produced through March 2003 but again was de-emphasized in all Apple marketing by then. There are probably few of these machines still around, and were never many to begin with in all likelihood. Some may have even sat in inventory for a long time--remember that IIe that showed up on eBay a few years ago that had been sitting at a dealer since 1993?