First question that comes to mind is: Have we seen the last of Dick Whitman?
"...after all, when it comes down to it, who's really signing this contract anyway."
More on it later... but in the meantime, how about using the great Tennessee Ernie Ford's "Sixteen Tons" for closing credits? The song, originally by Merle Travis, was Ford's biggest hit, topping the charts in 1955. The song was about workin' the coal mines, but it's the lyric, "I owe my soul to the company store," that's obviously of note here, with Don finally signing a contract with Sterling Cooper.
Previously: "Guy Walks Into an Advertising Agency" (Episode 3.06)