¥ with a single strikethrough, and an E at the end of currency?

I bought a book (early 2000s) and noticed that instead of the price being listed as ¥999, or 999円, it's listed as ¥999E. The "¥999E" version of the yen symbol only has a single strikethrough and not the double. The "E" is a typical English looking E, nothing else special about it.

Does the single strikethrough version mean something different? Why and what is the "E"? Was it just a typeset limitation with the manufacturer/printer?