Not every letter from every pulp writer that survives has been published; many remain on the open market and in private hands, coming up for sale from time to time...and they have stories to tell about Robert E. Howard.
The letter was posted on Facebook in Sep 2016 by Bob Meracle, who wrote of the acquisition:
One of the "lots" August Derleth Offered to sell to me (and I gladly snapped it up) was a collection of manuscripts which included 3 signed typewritten Conan stories. I sold the 3 a couple decades ago, but held onto this cool note that was sandwiched between them.
While not explicitly stated, these typescripts were likely originally from the collection of R. H. Barlow. In 1932, Barlow solicited manuscripts and typescripts from Robert E. Howard, H. P. Lovecraft, C. L. Moore, and other pulp writers, and Howard responded by sending several early typescripts for stories. Barlow’s receipt of these typescripts is mentioned in his 1933 diary, as well as in surviving letters from Howard. (CL 2.519; 3.47, 219) After Barlow’s death, his mother sold his collection.
The identity of the recipient is unknown; the name on the letter, although effaced, is too long to be "Barlow," and we know Howard sent Barlow a letter dated the very next day (14 June 1934, CL 3.215), so it is unlikely that Barlow was the recipient. So we are left with only the internal evidence of the letter. The reference to a request for a snap-shot recalls Barlow’s correspondence with H. P. Lovecraft in late 1933, although this might be coincidental. (OFF 78, 81) The reference to turpentine camps and voodoo is thus the primary clue.
“Turpentine camps” were work camps, largely employing black labor, including leased convict labor and sometimes workers held in debt bondage (i.e. charging them for food, clothing, etc. more than their wages could supply). These workers distilled turpentine from the resiny pine forests in the southern United States; during the 1930s their geographic range extended from North Carolina to Louisiana near the Texas border, with notable operations in Georgia and Florida. Zora Neale Hurston visited such camps to collect folk songs, magical recipes, and stories, some of which were published in academic articles and her collection Mules and Men (1935).
This was part of a general trend of anthropologists and collectors of ethnic music and folklore visiting prisons, work camps, and remote communities in the 1930s to record this material before it was lost—including a friend of R. H. Barlow.
Well, well—& so a friend of yours, like William B. Seabrook, has come into first-hand contact with the horrors of Damballa & his serpents. Who knows what waddling nigger washerwoman may not be a potent & dangerous mamaloi with power to evoke nameless horrors & send hideous zombis stalking through the land!—H. P. Lovecraft to R. H. Barlow, 21 Oct 1933 (OFF 83)
Thanks tremendously for the voodoo report, which I've read with extreme interest. your friend seems to have been quite an amateur Wm. B. Seabrook—& the experience must have been powerfully moving in its way. Later on, if you ever make a copy, I certainly wouldn't mind a spare carbon. Those "geachi" blacks must be rather an interesting study.—H. P. Lovecraft to R. H. Barlow, 13 Nov 1933 (OFF 85)
That voodoo encounter surely was picturesque—I'd hardly care to get into such close quarters with a crowd of excited blacks, but anthropological zeal will carry one far. So the "geechis" owe their superiority to insular isolation! I believe that, in general, all the Carolina island negroes are called "gullahs", & that their dialect differs from that of the mainland blacks. No doubt the geechis are a variety of these.—H. P. Lovecraft to R. H. Barlow, 29 Nov 1933 (OFF 88)