This is a perfect example of the Internet at work. I do not recall if I've ever seen a beige Mac Plus ED, but I do know I have seen pictures of a Platinum Mac Plus ED. In fact I'm pretty sure this is one:
That said, the popular theory is that the Mac Plus was Apple's most powerful Mac and as such commanded the highest price. Though there were educational discounts they did not approach the reduced cost of the ED which was an institutional buy only for which the Mac Plus was thought not initially available. The 512Ke therefore served as the only ED model when the Mac Plus was introduced. Now it's certainly possible at some point in 1986 the beige Mac Plus became the ED, but it has been traditionally speculated that the Mac Plus did not become an ED until after it turned Platinum and the SE and Mac II were introduced as Apple's most powerful MacS, which of course began the decline of the 512Ke except on the educational market, and then mostly outside the US.
So are you saying you actually have a beige Mac Plus ED, and not just a yellowed Platinum one? Is the copyright date on the back 1986 or 1987?