Stamp folding $s(21)$

The number of distinct ways to fold a strip of $21$ labeled stamps.

Updates

• Lower bound: $6927964218$ (6,927,964,218)
  Reference unknown
  [via Number of ways of folding a strip of n labeled stamps. - OEIS]
• Upper bound: $6927964218$ (6,927,964,218)
  Reference unknown
  [via Number of ways of folding a strip of n labeled stamps. - OEIS]