Someone say my name?
THM 1-5 are certainly a source of controversy. There are a good number of folks who have been looking far longer than me who have never seen one, most would tell you they are a myth and never existed.
There's actually a bit of fairly specific information lurking around that would lend some credence to the possibility that they did in fact exist. Rumors of the books abound like Bigfoot, but nobody can seem to come up with a photocopy or even a photograph of any of them.
Treasure Hunter Manual 6 mentions all of the 1-5 versions several times, enough to set the tone that each of the books had it's own theme. According to Journal of Discourses (JoD), all 5 of the books were printed in Chicago in the late 1950s and early 1960s. This is interesting because, unless I'm mistaken, this would have been the same time Karl was in California and was printing the original national Prospector's Gazette (Ironically enough, Volumes 1 through 5 which he began publishing in 1954). Karl hadn't attained the notoriety he would later find but a review of JoD also shows a great number of other books Karl indicates were published but for which there is no record of.
My thoughts on the topic are the until a copy turns up, nobody will know for certain. Even finding one copy of any of them would give credence to the others existing as well. Karl does not mention ISB numbers for any of these titles in JoD which tells me there is a high likelihood that they were independently published which means tracking them down would be nearly impossible through conventional means. He does indicate they were printed (most of them anyway) by Audubon Publishing in Chicago, though they are no longer in business. Having access to their records might shine some light on the topic, but this likewise seems an unlikely possibility. It doesn't appear that the books were registered with the Library of Congress (hence no ISB) and my guess is that if it was self-published, the print runs would have been very small as Karl would have been paying out of pocket for them and didn't have the "name brand" to push tons of copies (yet).
Some have suggested that Karl combined those volumes to make Volume 6. By then he was getting a head of steam reputation wise and was affiliated with Johnny Pounds (the Treasure Hunter), Art Lassagne (The Gold Bug) and Bill Mahan (Treasure World). I believe all would have been based in California at the same time (1960-1965-ish) and frequently cross promoted each other's material in their own periodicals. Lots of "scratch my back I'll scratch yours in exchange for advertising and all of these magazines and newsletters were staples well into the mid 1970s when the bottom started falling out of treasure publishing. This is also the same time when Treasure Publications started popping up. Prior to this, treasure stories were a by-product of the Western (True West, Golden West, The West, Frontier West, etc.) magazine trade as well as Men's magazines (Argosy, Saga, Cavalier, many others).
That's a lot of writing to essentially answer a question with, "Nobody really knows." Thanks for indulging me...
Randy