Representation of 5 types of pieces by its suit was well distributed for dragon Kings, wind Queens,wan Rooks, wheel Knights and bamboo Bishops. The 9 sets by Daniel Choi is shown below with pawns as blanks.
Other possible representation of 6 pieces is by positioning of tiles as noted by Justhalf and applying the faced down pawns above, the regular set looks ...
First, each Mahjong set contains the following 146 pieces:
4 each of Bamboo tiles numbered 1-9 (e.g., 4 Bamboos with a 1, 4 Bamboos with a 2, etc.)
4 each of Wheel tiles numbered 1-9
4 each of Wan tiles numbered 1-9
4 each of Wind tiles labeled E, W, S, and N (e.g., 4 East, 4 West, etc.)
4 each of Dragon tiles labeled C, F, and P
1 each of Flower tiles ...