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Seems like (but needs checking that) $$ \sum \frac{1}{\ell!} s(\ell,k) z^\ell t^k = \exp(t \sinh(z)). $$ That could probably be used to find other formulas, recurrences, etc.

ADDED: http://oeis.org/A136630 OEIS sequence A136630 is about these numbers.

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Seems like (but needs checking that) $$ \sum \frac{1}{\ell!} s(\ell,k) z^\ell t^k = \exp(t \sinh(z)). $$ That could probably be used to find other formulas, recurrences, etc.