C.C. Blatchley claims that Joseph and the 3 Witnesses informed him that the world must take their word for the existence of the plates.
C.C. Blatchley, "Caution Against the Golden Bible," New-York Telescope (New York, New York) 6, no. 38 (February 20, 1830): 150
In the Investigator, No. 12, Dec. 11, I published, by way of caution, a letter of Oliver H.P. Cowdry, in answer to my letter to Joseph Smith, Jun. Martin Harris, and David Whitmore—the believers in said bible of gold plates—which they affirm they have miraculously, or supernaturally beheld. I sought for evidences, and such as could not be disputed, of the existence of this bible of golden plates. But the answer was—the world must take their words for its existence; and that the book would appear this month.