I dont think the file is about her brother. Jack said it tells what she was doing in June (?) 1944. Peggy didn't enter the SOE until after Michael's death, and Captain America: The First Avenger takes place in 1942-1943, at which point Peggy was already with the SSR (I'm assuming as a liason from the SOE, but I dont remember if they explained it).
So even if Jack was confused by the initials, the timeline wouldn't have fit for her brother.
So even if Jack was confused by the initials, the timeline wouldn't have fit for her brother.