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[Linked Image from littlestuffedbull.com]

When Kirby made his departure from Marvel to DC in 1970, this was the first ad that heralded his arrival at DC.
It ran in PHANTOM STRANGER 8, July-August 1970, and other DC titles that month.
https://viewcomiconline.com/supermans-pal-jimmy-olsen-v1-130/
https://viewcomiconline.com/adventure-comics-1938-issue-395/

I was amused at how Kirby's arrival at DC was announced, almost as if it were the second coming of Christ. And they don't even say Kirby's name, I thought maybe because the DC contract papers were just signed, and Kirby hadn't announced his departure yet to Marvel.
Kirby's first issue for DC was 3 months later in JIMMY OLSEN 133, cover-dated Oct 1970.
https://viewcomiconline.com/supermans-pal-jimmy-olsen-v1-133/

Followed by JIMMY OLSEN 134, Dec 1970.
And then FOREVER PEOPLE 1 and NEW GODS 1, both Jan 1971.