The Avengers #70 - Enter the Squadron Sinister! (Issue)
Battle Covers: Avengers #53 (6/68: John Buscema), Avengers #70 (11/69: Sal Buscema), Fantastic Four #73 (4/68: Jack Kirby). There are probably more like this that I'm missing ... ? : r/comicbookcollecting
Avengers 70 (1969) Kang 1st full Squadron Sinister Appearance Fine+ Disney | eBay
AVENGERS #70. CGC 5.0! origin of the SQUADRON SINISTER, Grand Master & KANG | eBay