Here in the printer’s manuscript, Oliver Cowdery initially wrote “in the seventy and fifth”. Somewhat later he supralinearly inserted the expected year with slightly heavier ink flow, perhaps after redipping his quill. There are no examples in the original text of the specification for a year without the word year unless the word year appears earlier in the same sentence. For a list of cases where year is properly ellipted in the original text, see under Helaman 3:3. All the evidence elsewhere in the text argues that the word year was in the original text for Helaman 11:6 and also in 𝓞, no longer extant here.
Summary: Accept in Helaman 11:6 the corrected reading in 𝓟, “in the seventy and fifth year”.