It’s actually 8x8
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I found these symbols from Unicode’s Enclosed Alphanumerics block:
①②③④⑤⑥⑦⑧⑨⑩⑪⑫⑬⑭⑮⑯⑰⑱⑲⑳
Sensei’s Library have a script to convert move notation into small images, eg. W3 → ③, by means of which they can display black stones as well.
“Black stones” appear to be encoded from (11) to (20), but they display as white on Discourse’s dark theme so more likely they’re technically “solidly coloured”.