Peggy getting slowly more annoyed by the Americans she is surrounded by and their collective inability to use the letter U.
Steve writing her notes in which he adds a U after every O. Even the ones where there is already a U:
Dear Agent Carter,
Sourry abouut my ignourant American English. I’ll try tou dou better froum nouw oun.
Peggy retaliating by writing him notes without the letter U at all:
Dear Captain Rogers
Yo’re not fcking fnny.