Domino

After cataloging all 27 dominoes in the image and comparing them against the full double-six set, the [6, 1] domino is the only one missing.

The excluded domino is number 2 and 1

By systematically checking all 28 possible dominoes in a standard double-six set, I identified that the domino [4, 5] is not present in the image. All other dominoes from [0, 0] through [6, 6], including all combinations where each number appears with every other number from 0-6, are accounted for in the visible tiles.

By systematically cataloging all visible dominoes in the image and comparing them to the complete set of 28 dominoes (0-0 through 6-6), I identified the missing tile. After accounting for all visible dominoes, the [0,6] domino (blank/six) is not present in the image.

After checking the 27 tiles shown and comparing against the full double-six set, the only pair not present is 3 and 6

I identified all dominoes in the image by counting the pips on each tile. The complete double-six set should contain 28 unique tiles. After cataloging all visible dominoes, I found that every domino from the standard set is present except for the double-three [3,3]. The doubles present are [0,0], [1,1], [4,4], [5,5], and [6,6]. The domino [3,3] with three pips on both sides is absent from the arrangement.