I’ll take a wild stab at this. H=7mm Remington Magnum
Maybe during the time period this rifle was manufactured, whoever owned Savage at the time was jumping on the Mossberg bandwagon and adding the factory chambering to the model number by means of an additional letter to aid in providing the correct replacement parts in the future. It’s a wild stab, but there is evidence in the thread that another 7mm Remington Mag has the same model designation. I’ll even guess that there are additional 270 Winchesters out there with a D suffix......