Two Symbols
Two digits, an entire story.
A number of three or more digits built from exactly two distinct digit values, like 121, 1122 or 100001. Both must actually appear — using only one is Solid’s job.
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There are 4,537 of them, about 0.45%. The digit string is effectively compressed into binary: two symbols have 2^6−2=62 arrangements in six slots, but symbol choice and the no-leading-zero rule sieve that down to a few thousand.
It nearly coincides with distinct-2 in the Digit Makeup family (4,618 members): the only difference is two-digit numbers — the 81 two-digit numbers with differing digits count as distinct-2 but fail this feature’s three-digit floor.
Two-symbol numbers carry a cipher aesthetic: 100001 reads like a telegraph code and is also a palindrome, quad and Balanced; 777000 proves minimal vocabulary and flamboyance can coexist.