Well, Doug, I'm to the point of saying the survey is done so far as I expect to be able to take it, barring any lucky further discoveries.
Here are my conclusions:
a.Thirteen paperbacks referred to Tolkien and/or The Lord of the Rings on front or back cover.
b.Lancer led the way with Tolkienian marketing, using it on 5 of their books.
c.An American fan of Tolkien just looking at paperback cover blurbs would be led to the following:
--5 works of genuine high fantasy for adults: Eddison's Worm Ouroboros and Mistress of Mistresses, Morris's Wood Beyond the World, Dunsany's King of Elfland's Daughter (all Ballantine); Pratt's Well of the Unicorn (Lancer);
--2 Tolkienian fantasies for children: Garner's The Weirdstone of Brisingamen and The Moon of Gomrath (Ace)
--4 works of swords-and-sorcery: Howard's Conan the Adventurer and Conan the Conqueror (Lancer); de Camp's Tritonian Ring (Paperback Library); Moorcock's Jewel in the Skull (Lancer)
--1 work of fantasy that I'm not prepared to put into any of the categories above: Hubbard's Slaves of Sleep (Lancer)
--1 work of science fiction: Herbert's Dune (Ace)
It seems to me that publishers were slow to distinguish Tolkienian fantasy or high fantasy from other kinds of fantasy. Put another way, I suspect that American paperback publishers clearly differentiated science fiction from fantasy, but that was as fine a distinction as they were prepared to make for several years.
Publishers seem to have been a bit slow to catch on to the idea of multi-volume fantasy cycles. Ballantine's Eddison books are referred to as a "group." Lancer's Jewel in the Skull is heralded as "the first of a series destined to rank with... the Lord of the Rings trilogy." As un-Tolkienian as this book (and its author) were, it might be considered to be the one that comes closest to alluding to the idea of a multi-volume fantasy work with continuing characters and so on; or it might be "tied" for this with Eddison. (NB I haven't seen the two middle books yet; I don't think they allude to Tolkien, but perhaps they do.) To my eye, the cover design of the Eddison books is most "Tolkienian" (in so markedly looking like Ballantine's Tolkien set).
With regard to the distinction in the 1950s and 1960s between fantasy and science fiction, I'm not sure American publishers were any clearer than British ones. Remember the blurb (by Naomi Mitchison) on the original edition of The Fellowship of the Ring that called it "super-science-fiction"? I've always thought that an odd description, and recently came across a good contemporary explanation of this usage. In Basil Davenport's Inquiry into Science Fiction (1955), he wrote:
Recently there appeared a book called The Fellowship of the Ring, which, though intended for an adult audience, is pure fairy tale; its characters are elves and enchanters (and not a hint of a gene or a chromosome among the lot) but even that carries on the jacket a quote saying, "This is really super-science-fiction." That is, of course, sheer nonsense; it is hard to know where to draw the line defining science fiction, but it certainly excludes The Fellowship of the Ring. What the critic actually means is, "This is imaginative writing, but you needn't be ashamed to be seen reading it." (pp. 79-80)
Basil Davenport (1905-1966) was one of the first heavyweight literary critics to proclaim the worth of science fiction as literature. He served for many years on the Editorial Board of the Book-of-the-Month Club, and published his short book on science fiction in 1955, five years before Kingsley Amis's New Maps of Hell: A Survey of Science Fiction (1960). One of Davenport's first books was a companion to Austin Tappan Wright's Islandia, An Introduction to Islandia (1942). He also introduced the omnibus of five novels by Olaf Stapledon, To the End of Time (1953) and edited some anthologies of science fiction and horror.
An interesting sidelight of Davenport's comment is the idea that Mitchison meant Tolkien's work to be seen as imaginative writing (super-science-fiction)of a type that one needn't be ashamed to be seen reading---whereas the implied derision is that one should feel ashamed to be seen reading regular science fiction. Such bone-headed attitudes persist to this day, but are thankfully less common. Even Margaret Atwood has finally admitted that she writes science fiction!