I am happy to note that no less a reader of Heidegger than Derrida considers it (in Positions) “the most continuous and most difficult thread of Heidegger's thought.” Why continuous? Well as far as I can ascertain it first appears in the 1919 Kriegsnotsemester (according to Kisiel) and we find it, of course, all over the Beiträge zur Philosophie (Vom Ereignis) where we find such wonderfully dense sentences that almost feel like scripture: “In enowning, enowning itself resonates in counter-resonance.”
It continues through various hints of its importance through the fifties, until, in his last major lecture, he comes out and notes its importantance. The Beiträge wasn't published until after he died, so for decades there was a public (lectures) and a private (manuscripts) Heidegger. The two have only merged in recent decades.