From the chapter "Ereignis: (Only) Another Name for Being Itself" in Richard Capobianco's excellent Engaging Heidegger.

C. 'Die Überwindung der Metaphysik,' GA 67 (1938-9) and 'Die Geschichte des Seyns,' GA 69 (1938-40)

Volume 67 of the Gesamtausgabe, titled Metaphysik und Nihilismus (published in 1999; not yet translated) includes the manuscript 'Die Überwindung der Metaphysik' ('The Overcoming of Metaphysics'), which belongs to the group of writings related to Beiträge. In this collection of meditations, Heidegger makes similar references to the relation of Ereignis and Beyng. For example, he refers to 'the echo of beyng as Ereignis' (GA 67, 99), and he explicitly states:

Ereignis ist das Seyn. (GA 67, 62)
Ereignis is Beyng.

The two manuscripts collected in GA 69 (published in 1998; not yet translated) are also related in content and style to the meditations of Beiträge. The first (and longer) piece is titled 'Die Geschichte des Seyns' ('The History of Beyng'), composed 1938-40; the second is 'Koinon: Aus der Geschichte des Seyns' ('Koinon: From the History of Beyng'), which dates from 1939-40. In the first composition, we find a number of relevant and revealing epigrams, among them the following:

Das Er-eignis. (Seyn). Austrag. (GA 69, 28)
Das Er-eignis. (Beyng). Drawing out.

Das Seyn ist - Er-eignis. (GA 69, 106)
Beyng is - Er-eignis.

Das Seyn ist Er-eignis. (GA 69, 134)
Beyng is Er-eignis.

Das Ereignis als das Seyn.
Das Seyn als die Wahrheit.
Das Seyn nur ist. (GA 69, 141)
Das Er-eignis as Beyng.
Beyng as truth.
Only Beyng is.

Aber wie 'ist' das Seyn? Das Ereignis. (GA 69, 144)
But how 'is' Beyng? Das Ereignis.

Got it?  ¶ 7:51 AM