Strike Team Delta is from the first Avengers film. There's a point where they show some footage of Clint and Natasha from an operation in Abidjan and it labels them that way. It's also included in their SHIELD dossiers that were part of the fancy Phase I bluray set.
SHIELD file Clint.jpg SHIELD file Natasha.jpg
Sorry they turned out sideways, I dunno how to fix that.