HACD from the Hacash 2018 anonymous white paper, Block Diamond.
Each of HACD is a unique 6-letter inscription randomly generated from a 16-letter alphabet(A, B, E, H, I, K, M, N, S, T, U, V, W, X, Y, Z).
"NHMYYM" made history as the first HACD, minted on 2019-05-16.