DF is for “Derivata Fiat” (Fiat derived) and that simply means that this expensive (then and now) car was based on the reliable and rock-solid Fiat 1900 platform.
Of course when it was unveiled at the 1953 Geneva Salon, it was not just a Fiat 1900 with a different body as Cisitalia clients were used to something more than a coachbuilt body, so the engine was modifed with a diffeent head, camshaft and a couple of carburetors instead of only one. This rised the power to 85 hp, a very good number in 1953.
This particular car is said to be the only one survived which is in perfectly working conditions. Actually, it looks perfect under anu point of view: it has been clearly professionally restored in Ohio, U.S.A., at a cost of $230,000 and now it’s on the market here at €268,000 (today $322,000).