Costco Car Battery Failed at 21 Months

I purchased an Interstate group 78, (model number on battery looks to be 850263), 800 CCA battery purchased 3/30/2015 and now the battery won't hold a charge after 21 months. Costco's warranty is 42 months. Heading to Costco for a free replacement. I can't find this battery on Interstate's website and am wondering if the batteries Costco sells is a substandard version?