I'm not clear whether you need "low stress" stamps or not, but for marking our galvanized steel we use an impact gun from Hilti with the stamp head that allows several combinations of letters(8 spaces, I think) that is usually enough to mark our piece marks, shop order, sequence#, and column direction. All of that info takes 3 strikes, but it is much faster than stenciling with single hand stencils hit with a hammer.
Our DX750 with stencil marking head attached.
Stencils:
Not sure if they have low stress stencils, so keep that in mind if interested in this marking system. link below:
http://www.hilti.com/holcom/page/module/product/prca_catnavigation.jsf?lang=en&nodeId=-17075edit:...added pics of the stencils