When you see it your mind will be blown...
Do you see it yet? IT'S SOUSA. From the 1940s Agent Sousa! In New York City in the year 2012!!! Looking... good.
To heck with the army, does PEGGY know what's going on?
Too deep Souza. Tone it down.
The obvious explanation is that Marvel liked the actor and found a place for him on Agent Carter. But what's the in-universe explanation for his youthful appearance, despite the fact that he looked OLDER in the 1940s?
Weirdest explanation wins an air guitar.
I fully expect you to serenade us all during the commercial breaks while we're watching the episode tonight.