Wynn gives an example of a balanced incomplete block design for 24 varieties in 56 blocks of three plots each which involves only 24 distinct blocks. It is shown that this design can be obtained from a hierarchic group divisible association scheme, and is one of a set of eight possible designs.
"A Note on a Finite Population Plan." Ann. Statist. 6 (3) 697 - 699, May, 1978. https://doi.org/10.1214/aos/1176344213