Rattletrap is the Beckfoot car of the Blacketts, described as a smallish, ancient motor-car with badly dinted (sic) mudguards and a luggage grid at the rear (PP1). Mrs Blackett turned the car round in a series of short dashes in the Tysons yard: Oh well, she said as she stalled the engine and got out and looked round the car: That front mudguard was dinted already several times. And that one at the back’s always unlucky. There’s no real harm done (PP17).
In The Picts and the Martyrs there is a picture of Rattletrap with the Great-Aunt sitting in the rear seat holding a parasol upright . Billy Lewthwaite drives her along the Lake road to Swainsons until he runs out of petrol (PM25). Rattletrap is crank-started: ... Billy, whirling the rusty crank, stirred old Rattletrap to life (PM25).The car depicted (PM26) is clearly a Trojan (see photo on this page), although the Trojan has only two forward gears, raising an anomaly; at the end of Pigeon Post Captain Flint drives her up to the camp: Rattletrap, knowing her old master, started off as if she meant it. They swung through the gate and sharp right into the road. The gears changed, second … third. Captain Flint said Hold tight, Dick ... She won’t do forty except down-hill, but she's a bit of a broncho round corners.... (PP33).
However, AR has clearly depicted a FIAT 501 (PM 26) rather than a Trojan. The Trojan does not have three gears, has no hand-crank (it is started from within the car by a lever which connects directly to the engine below the driver's seat - its 'radiator' and 'bonnet' are just for show) has solid disc wheels, not spoked 'artillery' ones such as AR has drawn, and a top speed of 35mph whereas the Fiat could 'do forty' easily; the Trojan carried its spare wheel on the side, not the rear, and had no luggage grid. The Fiat 501 was built from 1919 and looked exactly as that drawn by AR: it had artillery wheels, a spare wheel on the back and a luggage grid (the spare could also be side-mounted), a sloped windscreen, front seats that rose in a ridge above the bodywork, a horseshoe shaped radiator, three forward gears, and it was very good on hills too - the Trojan with its two forward gears and lack of speed was not designed for hilly districts whereas the Fiat was. The 501 was built under licence in England during the 1920s and was a popular car. AR may have driven a Trojan, but Captain Flint clearly preferred a Fiat!
The Turner/Blackett family don't appear to have a car before the events described in Pigeon Post; in Winter Holiday Molly Blackett uses a hire car to visit Holly Howe to discuss Nancy's mumps (WH9). Later, Captain Flint arrives at Beckfoot by hire car, which waits for him while he talks to Molly before setting off for the Fram.