Perhaps you have added to your pleasure in watching Maggie Smith and Jim Carter -- and the others too who act in Downton Abbey -- by noting the mistakes that Julian Fellowes makes in his script when he has characters, in 1915, or 1920, use words and phrases that came a half-century later or that never came to England at all.
Two that stand out:
1) Shirley MacLaine, as Mrs. Levinson, the American heiress, who says "I'm an American. Have gun, will travel." That last is the title of a famous series, starring Richard Boone as a Good Gun For Hire, that ran on American televisoion back in the late 1950s and early 1960s, to the delight of young boys, after Fess Parker had run his course as Davy Crockett king of the wild frontier, and about the same time as Gunsmoke with Marshall Dillon, Chester, Miss Kitty, and Doc, on Saturday nights, while Sunday was when James Garner appeared as both Brett and Bart Maverick. "Have X, Will Travel" may have been, was used much earlier in the century, but the verison made known by Paladin on his calling card --" Have Gun, Will Travel" -- is not a phrase that would have been in use c.1920.
2) In the latest episode, that ran last night in Eastern North America, Branson, having been noticed by, and noticing, the very good-looking new maid, Edna, tells Mrs. Hughes that in such entanglements, and how to avoid them, he was on a "learning curve." "Learning curve"? In the early 1920s?
So in Season Four, which I suppose will take us up through the Jazz Age (with Rose charlestoning all over the place), and and perhaps right up to or beyond the Depression, someone is likely to talk about new plans to make Downton Abbey a "cash cow." That's a phrase from Bruce Henderson, of BCG (the Boston Consulting Group), circa 1965-1970, and not earlier. And "cash cow" is, to judge by the NPR segment about that venerable pile, Downton Abbey itself, that was televised following the last episode, exactly what it has become, thanks to the series, for its inhabitants, the current easygoing Earl of Carnarvon and his reindeerishly-sweatered wiife, the Countess of Carnarvon.