Bookends
The first and last digits shake hands.
A number of 2+ digits whose first and last digits match, like 707, 9009 or 123321. The middle can be anything — the two ends must echo.
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Bookends carry the lightest constraint in the symmetry family: exactly one pair of digits is locked. Among k-digit numbers the first digit has 9 choices, the last is forced to follow, and everything in between is free — so every digit-length class is exactly one-tenth bookends, totalling 99,999 across the range: 9.9999%, almost precisely one in ten. Modest rarity, modest EP.
It is the strictly weaker cousin of the palindrome: every 2+ digit palindrome is automatically bookends, but bookends demand nothing of the middle. Only 1,989 of the 99,999 bookends numbers are palindromes — a 2% pass rate; the other 98% are loose symmetry that only matched the ends.
The name comes from the bookends on a shelf: the books in between may lean however they like, but the two ends must come as a pair. Bonus trivia: the two-digit bookends 11, 22 … 99 are all multiples of 11 — the only length class with that perk.