A battered pot found in a garage in Cornwall, broken in antiquity and broken again and mended with superglue some 5,500 years later, was treasure – but the scruffy little cardboard label it held is now unlocking a lost history of finds from excavations in Egypt scattered across the world in the late 19th century.

The pot came with an odd family legend that back in the 1950s it was accepted in lieu of a fare by a taxi driver in High Wycombe. Alice Stevenson, curator at the Petrie Museum in London, which among its 80,000 objects has the original excavation records and hundreds of pieces from the same Egyptian cemetery, believes the story is true and may even have identified the mysterious passenger.

"I got the email from them on my second week in this job," Stevenson said. "I could hardly believe what they'd found. I literally jumped up and down in excitement. The pot is wonderful, a rare find indeed. The label is absolutely fantastic."

When Guy Funnell and Amanda Hawkins found the pot while clearing a garage stacked with his father's possessions, they made the connection with a BBC documentary they had recently seen, The Man Who Discovered Egypt, about the archaeologist Flinders Petrie.

Petrie's meticulous records and scientifically based excavations in the region transformed archaeology, and he created a timeline still in use today through sorting thousands of pots by date, enabling tombs, temples and entire towns to be dated from the fragments of broken pottery on the sites.

The little black and red pot was one of the few occasions when he was not only completely wrong, but admitted that, mortifyingly, a French rival was right.

Petrie, like his 19th-century contemporaries, sent back tons of material from Egypt to universities and museums funding his excavations, and later sold a huge collection that became the basis of the Petrie Museum – the most difficult to find in London, "temporarily" housed in an old stables on the UCL Bloomsbury campus alleyway since the 1950s, but with the finest collection of material from the region outside Egypt.

It was known that he gave pieces to individuals, at a time when a visit to a celebrity archaeologist's dig was the highlight of any tourist or VIP trip down the Nile. The little label proves this was done on a systematic basis not previously guessed at. It is a neat commercially printed card, with an Egyptianate border, boasting that the "Libyan Pottery" from 3,000 BC was discovered by Prof WM Flinders Petrie in 1894-5. The card was clearly one of many, but pot, card, and excavation record are linked by the faintly pencilled number 1754.

"There were obviously many such cards, but I have never seen or heard of one before – there must be more out there, which would help us trace the distribution of this material through museums and private collections," Stevenson said.

The pot is now being conserved for display at the museum, on loan for a year. Funnell dimly remembers it in childhood, and his mother remembered the story that his grandfather, Charles Funnell, was given it to settle not just one but several unpaid taxi fares.

Stevenson believes the mystery customer may have been a curator at the Ashmolean Museum in Oxford, Joseph Grafton Milne, who died in 1951, but was recorded as visiting Petrie in Egypt in the 1890s. The link between the distinctive pots: the Ashmolean has a bowl from Milne's collection from the same grave as Funnell's pot, and she thinks it is probable Milne obtained both from Petrie.

The Petrie has some tiny shells, pierced as beads, and a piece of rock crystal from the same grave, but the records show many more pots came from the same grave, and thousands from the cemetery, and the card may help to trace them.

Petrie was wrong about the pots: they were Egyptian, not Libyan. He was fooled because their distinctive black fire-scorched rims were so different from the others he found. However, a French scholar, Jacques de Morgan, established that they were Egyptian but pre-Dynastic, 600 years older than Petrie first believed.

"It was one of the few occasions when Petrie was not only wrong, but admitted it publicly," Stevenson said, "a very unusual occurrence."

After conservation work, the treasure from the Cornish garage will go on display next month, a scruffy star of the museum's Festival of Pots.