Nomenclature Changes
In 1944-5 the Germans changed their system of artillery designations from the old "year" system. Each weapon was to have a number showing their caliber group, a letter denoting the ammunition group, and the last two digits were from the weapon drawing number. In this case 10 denoted 100 mm caliber using the H group of ammunition.
Read more about this topic: Panzerwurfkanone 10H64