'The variety of stuff on offer is amazing,' says Michael Ware, the museum's curator since 1966. 'Finding gems amid the junk is the name of the game. It's quite possible for someone who knows what's what to pay maybe a fiver for something as rare as a Bugatti Type 35's sidelight.
'My list tends to involve things such as old petrol cans and posters. Right at the other end of the scale, 120 complete cars are for sale in the Automart that goes hand-in-glove with the Autojumble. They include a 1926 Model T Ford, a 1936 Humber Hawk and several E-type Jaguars.'
This evening's big attraction at Beaulieu is a Christie's sale of unrestored and collector's cars. Bentleys and Rolls-Royces, a delectable pre-war Delage and a vast Lincoln Continental share the 84-lot catalogue with nothing more plutocratic than a tiny Isetta 'bubble car' and a 33-year-old Volga saloon from Russia.
'An opportunity for members of the car fancy (sic) to obtain long desired items and to disencumber themselves profitably of impedimenta' is how the museum advertised its first Autojumble in September 1967, which attracted 76 stalls and 4,933 people. What started as a toe-in-the-water venture has been a two-day event since 1980.
The inspiration came from the 'swap meet' held each year in Hershey, Pennsylvania, the world's biggest gathering of the classic car clan, with 10,000 stalls and 250,000 visitors.
'I had never been to Hershey, but reading about it made me think that something similar might work over here,' Mr Ware recalls. 'Inventing the word 'autojumble' is my only claim to fame. We should have taken out a copyright, because dozens of autojumbles are now staged up and down the country. The big difference is that virtually none of them has more than 200 or 250 stalls.
'Being the first in the field - and therefore having several years to become established - is one of the main reasons for Beaulieu's success. Having a full-time staff to plan and run the weekend efficiently is another big advantage. For instance, if it pours with rain and cars get bogged down, there's an army of tractors standing by to pull them out.'
The late Michael Sedgwick, one of the National Motor Museum's most stalwart supporters, used the public address system to enliven early Autojumbles with announcements. The idea was to put buyers in touch with sellers.
'Michael's pithy 'want ads' were very amusing and frequently produced results,' Mr Ware says. 'But there were times when the noise was a bit too much for some stallholders. We used to find old socks stuffed down the loudspeaker trumpets.'
In terms of visitors, the Autojumble is the biggest event in one of the museum's landmark years. It was in 1952 that young Lord Montagu of Beaulieu, faced with a major book-balancing problem, risked the wrath of his ancestors - three of them kings - by going into the stately home business. The three veteran cars on display were a tribute to Lord Montagu's father, one of Britain's earliest and most influential motorists, who is also remembered for having an affair with Eleanor Thornton, the Edwardian beauty who inspired Rolls-Royce's lyrical Spirit of Ecstasy mascot.
The old cars formed the nucleus of the fast-growing Montagu Motor Museum. Determined to preserve it for posterity, Lord Montagu took steps that resulted in its re-creation as the National Motor Museum, run by a charitable trust, which opened in 1972.
Mr Ware, who was a photographer, joined the staff in 1963. By chance, his letter landed on Lord Montagu's desk the day after Beaulieu's photographic librarian handed in her notice. Lord Montagu had petrol in his blood, but Mr Ware's passion for old cars was acquired rather than inherited.
'We always had a car, because my father was the 'traveller' for the family's leather business, but he wasn't an enthusiast,' he says. 'We lived in Devon and my interest was sparked by an uncle who always turned out to watch the Exeter and Land's End trials.
'My first car was a 1931 Austin Seven. It was over-priced at pounds 35. I teamed up with another fanatic and we made our competition debut in the 1960 Exeter Trial. We had our picture in Motor Sport and thought we were famous.'
Mr Ware, who relies on a Vauxhall Cavalier automatic for day-
to-day transport, now describes himself as 'ringmaster, nursemaid and nanny' to just over 300 cars, motorcycles and commercial vehicles. They span almost a century, from the unique Knight of 1895 to the last right-hand-drive Citroen 2CV. His favourite is the 1915 Prince Henry Vauxhall. 'It looks wonderful, handles well and is great fun to drive.'
The popular 'Royal Family Motoring' exhibit is a reminder that the museum's 40th birthday coincides with the 40th anniversary of the Queen's accession. The cars include a unique Reliant Scimitar, the Metro driven by Lady Diana Spencer before she became the Princess of Wales, and the Austin Maxi in which Prince Michael of Kent tackled the 1970 World Cup Rally from London to Mexico City.
Another of this year's special attractions is the mind-boggling Chitty Chitty Bang Bang built for Count Louis Zborowski in 1921. Powered by an 18.9-litre Benz engine, the chain-drive monster raced at Brooklands before taking Zborowski and his wife as far afield - for those days - as the Sahara.
On loan from an American museum, it stands alongside one of three 'magic' cars built for the Chitty Chitty Bang Bang film. Ironically, the movie version attracts more attention than the original that inspired Ian Fleming's story. Elsewhere, the fact that an identical car was featured in Back to the Future is what attracts young visitors to the De Lorean.
'The real Chitty got its name from a bawdy song that was popular during the Great War,' Mr Ware says. 'The infuriating thing is that I don't know the words.'
The Beaulieu International Autojumble's reputation suggests that he may well have found a dog-eared copy of the lyrics by tomorrow evening.
The National Motor Museum, Beaulieu, Hampshire S042 7ZN is open every day from 10am-6pm from May to September and 10am- 5pm from October to April. Admission: pounds 6.75 adult; pounds 4.75 child; pounds 5.25 students and senior citizens. There is no extra charge for the Autojumble weekend.