identifying expansion series

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Consider $$ f(x;a)=\sum_{k=0}\frac{x^k}{k!}\prod_{i=0}^k[(1+a)^{i+2}-(i+2)a-1], $$ where $a>0$. Can the series in the right-hand side be reduced to (or is it) to a more compact form (e.g., a special function of some sort)?