4 page letter from Carl Belknap to Howard, not dated
Belknap had a letter published in the October 1933 issue of Weird Tales, but other than that, while he discusses several unsold stories he’s written (and separately having had two stories rejected by Weird Tales), he does not appear to have been published. Undated, but from internal references, appears to be from late 1933.
Source:
Windy City 2022 Auction Catalog.