A one-box-shift morphism between Specht modules

We give a formula for a morphism between Specht modules over $(\mbox{\rm\bf Z}/m){\CMcal S}_n$, where $n\geq 1$, and where the partition indexing the target Specht module arises from that indexing the source Specht module by a downwards shift of one box, $m$ being the box shift length. Our morphism can be reinterpreted integrally as an extension of order $m$ of the corresponding Specht lattices.