In a previous post on Rules Expansions, I mentioned a late 1980 Gateway to Adventure catalog that shows the Holmes Basic Set side-by-side with an unpictured but soon-to-be-released Expert Set. A screenshot of this is above, taken from a scan of a page posted by TheMilford at the Tome of Treasures in 2009. As you can see, it's actually just the booklets that are advertised rather the whole sets. (If anyone has this catalog, please let me know if there is another entry for the sets).
While the listing shows the cover of the Blue Book, it is given the product code 2014, which was used for the Moldvay Basic Set rulebook. Most likely the new rulebooks were in production but not ready for photography at the time this catalog was prepared. The same page also shows the first print of B2 (written for Holmes) rather than the revised B/X version, and also has an unpictured listing for the module B3 (the first module for Moldvay), "the newest of the instructional settings for the D&D Basic Set".
Still, if you saw this catalog in 1980 you would probably have assumed that the forthcoming Expert Set was going to be a companion to the existing Holmes Basic Set, rather than the significant revision with new cover artwork that eventually appeared in 1981. Earlier in the year (Dragon #35, March 1980), Gygax had revealed that "Design is now hard at work on the second boxed D&D game, the Expert Set. It will take players through at least 12th level of experience, tie in the best of the “Original” material, and actually add some new classes, spells, magic, monsters and so on." In context, this appeared to be an Expert Set that would complement the Holmes Basic Set. The above catalog entry is the closest TSR got to advertising this mythical Holmes Expert Set.
Does anyone remember seeing this catalog at the time? Later copies of the Gateway to Adventure Catalog with the same cover, including the one I have from a Moldvay Set I found at a Goodwill in the mid-80s, replace the above entries with the B/X Sets. The copy from the Tome of Treasures site was found with a first print Cook/Marsh Expert Set.