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Oh, no, it was almost certainly /a/ in patens*, but also in *latens* and *patiens. My original question, which I ... be pronounced differently from the other two in modern American English. What caused that split-off and when did it happen?

This is not an answer, but it occurs to me that, in addition to "pat. pending", another 'patent' usage that would have been householdy to Americans of an earlier day was "patent medicine" (archaic AmE for 'soda', wot?). And perhaps 'patent leather' as well.