GC S8/X2?

While flipping through issue #71 of Dragon (March 1983), I came across a strange little snippet of information that either overlooked before or that I'd simply forgotten. At the beginning of his article on the quasi-deities of the World of Greyhawk, Gary Gygax says the following:

From the very beginning, TSR's Dungeons & Dragons modules have carried alphanumeric codes that divided them into series. Each series was, with a few exceptions, focused on a specific subject matter or style of play, such as the B-series for low-level (beginner) characters or the D-series focusing on the machinations of the Drow. 

Starting with the release of The Forgotten Temple of Tharizdun in 1982, these designations started to become much less clear-cut. Tharizdun was designated module WG4, the very first module to carry the "WG" code, which presumably stood for "World of Greyhawk." Two more more modules with Gygax's byline followed with this same code (Mordenkainen's Fantastic Adventure and Isle of the Ape), but there were never any modules with the codes WG1–3. (It's my understanding that The Village of Hommlet is WG1, the never-made Gygaxian Temple of Elemental Evil WG2, and The Lost Caverns of Tsojcanth WG3, though why this is the case, I have no idea.)

All of that is confusing enough if, like me, you care at all about the minutiae of TSR's production and releases. However, in the excerpt above, Gygax refers to The Land Beyond the Magic Mirror as "module GC S8/X2." Let's leave aside the fact that the actual published version of the module carries the designation EX2. That still leaves me wondering what Gygax's stated designation of the module even means. "GC," I suppose, is a reference to "Greyhawk Castle," which makes some sense, given that, in his original campaign, there was a section inspired by Alice's Adventures in Wonderland. But what are we to make of "S8/X2?" 

In 1982, the S-series of modules already existed, but S4 was the highest-numbered entry. Likewise, there was an X-series but it was not associated with the AD&D line. Consequently, it seems unlikely that Gygax is referring to either of these series. I wonder if it might be the case that the numbers refer to the way he keyed Castle Greyhawk in his home campaign. If so, I'm not at all familiar with the way he did this and, absent that context, it's quite obscure.

If anyone has any insight into this, I'd love to know. In the grand scheme of things, it's a very minor mystery, but it's nevertheless one that has piqued my interest.