The process you are describing is GTAW-gas tungsten arc welding {AKA tig-tungsten inert gas} the rod should match the base metal so if it were stainless it would be a stainless filller. I will take an assumption it was mild steel therefore the filler should be ER70-S?. I'll break the glyphic down for you E=elcetrode, R=rod, 70= tensile strength70,000 psi of the weld material, S=solid, ? there will be a number here for the additional alloying elements in the filler material.
Good luck with the report,