David,
E71-T1C is the code for a 100% CO2 shielding gas wire. I did a quick search on line and couldn't find a ProStar product that had this AWS designation. Here's the catalog I did find.
http://ecatalog.praxair.com/wcsstore/prostar_eflyer_store/pdf/P-8469ATubularWire.pdfThey list a Pro star E71-TM, which is a 75/25 mixed gas product.
All that said, some of these E71 FCAW-G products can be used with either 100% CO2 or 75/25 Ar/CO2. Suggest you find a manufacturer's datasheet for your specific product. I'll second the recommendation that another poster made that you speak with the EOR. Shielding gas will alter the mechanical properties of the finished welds. Based on what I saw looking at similar Hobart product datasheets, it' appears that the tensile properties of the weld go up and the impact properties go down when you switch from 100% CO2 to mixed shielding gas. Your EOR may reject what was done on that basis, or because your welder isn't qualified on that process with the mixed shielding gas. Or just because he's having a bad day. Regardless, good luck.