Which of the two is more correct?
contributory capacity or contributive capacity
(referred to the capacity of a province in ancient Mesopotamia to contribute to state demand for foodstuffs)
If we're going to use NGram to make the case here, it would be wise to check specifically for usages with "capacity".
It's only my personal opinion, but I suspect that given contributory is a far more common word in general, it may be some usages here are erroneous for that very reason. I thought the correct form was contributive capacity before seeing the NGram, which seems to confirm my guess.